Rustic Sweet Potato and Caramelized Onion Galette with Gruyère

Quick Recipe Version (TL;DR)

• Yield: 6 servings
• Prep Time: 30 minutes
• Cook Time: 1 hour 5 minutes
• Total Time: 1 hour 35 minutes

Quick Ingredients

• 1 1/4 cups (150 g) all-purpose flour
• 1/2 tsp fine sea salt (plus more to taste)
• 8 tbsp (113 g) cold unsalted butter, cubed
• 3–4 tbsp ice water + 1 tsp apple cider vinegar (optional)
• 2 tbsp unsalted butter + 1 tbsp olive oil (for onions)
• 2 large yellow onions, thinly sliced
• 2 medium sweet potatoes (about 1 1/2 lb / 680 g), peeled and thinly sliced
• 2 cloves garlic, minced
• 1–1 1/2 tsp fresh thyme leaves (plus extra for garnish)
• 1/4 tsp black pepper + pinch red pepper flakes (optional)
• 1 1/2 cups (about 150 g) grated Gruyère cheese
• 1/4 cup (25 g) grated Parmesan cheese (optional)
• 1 large egg + 1 tbsp milk or cream (for egg wash)
• Flaky sea salt, for finishing

Do This

• 1. Make dough: Pulse flour, salt, and cold butter until coarse crumbs form. Add ice water (and vinegar) just until dough holds together. Shape into a disk, wrap, and chill 30 minutes.
• 2. Caramelize onions: Cook sliced onions in butter and oil over medium-low, stirring often, until deep golden and jammy, about 25–30 minutes. Stir in garlic, thyme, salt, and pepper.
• 3. Prep filling: Peel and thinly slice sweet potatoes (about 1/8 inch thick). Grate Gruyère (and Parmesan, if using).
• 4. Roll dough: On floured parchment, roll chilled dough into a rough 12-inch circle. Transfer parchment and dough to a baking sheet.
• 5. Assemble: Spread caramelized onions over the center, leaving a 2-inch border. Top with half the cheese, arrange sweet potato slices, then sprinkle remaining cheese.
• 6. Fold and brush: Fold dough edges over filling, pleating. Whisk egg with milk and brush crust. Sprinkle thyme and flaky salt.
• 7. Bake at 400°F (200°C) for 35–40 minutes, until crust is deep golden and potatoes are tender. Cool 10 minutes, slice, and serve warm.

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Why You’ll Love This Recipe

• Rustic, free-form tart that looks impressive but is very forgiving and approachable.
• Cozy flavors: sweet roasted potatoes, deeply caramelized onions, nutty Gruyère, and fragrant thyme.
• Great as a vegetarian main with a salad, or as a fancy side dish or brunch centerpiece.
• Easy to make ahead, reheat, and even customize with your favorite herbs and cheeses.

Grocery List

• Produce: 2 medium sweet potatoes, 2 large yellow onions, 2 cloves garlic, fresh thyme
• Dairy: Unsalted butter, Gruyère cheese, Parmesan cheese (optional), 1 large egg, milk or cream
• Pantry: All-purpose flour, olive oil, fine sea salt, black pepper, red pepper flakes (optional), apple cider vinegar (optional), flaky sea salt

Full Ingredients

For the Pastry Dough

• 1 1/4 cups (150 g) all-purpose flour
• 1/2 teaspoon fine sea salt
• 8 tablespoons (1 stick / 113 g) cold unsalted butter, cut into small cubes
• 3–4 tablespoons ice water, as needed
• 1 teaspoon apple cider vinegar (optional, helps keep the crust tender and flaky)

For the Caramelized Onion and Sweet Potato Filling

• 2 tablespoons unsalted butter
• 1 tablespoon olive oil
• 2 large yellow onions, thinly sliced (about 4 cups)
• 2 cloves garlic, minced
• 1–1 1/2 teaspoons fresh thyme leaves, plus extra for garnish (or 1/2 teaspoon dried thyme)
• 2 medium sweet potatoes (about 1 1/2 pounds / 680 g total), peeled and sliced into 1/8-inch rounds
• 1/2 teaspoon fine sea salt (plus more to taste)
• 1/4 teaspoon freshly ground black pepper
• Pinch red pepper flakes (optional, for gentle heat)

For the Cheese Layer

• 1 1/2 cups (about 150 g) coarsely grated Gruyère cheese
• 1/4 cup (25 g) finely grated Parmesan cheese (optional, for extra savoriness)

For Assembly and Finishing

• 1 large egg
• 1 tablespoon milk or heavy cream
• Flaky sea salt, for sprinkling
• Extra fresh thyme leaves, for garnish

Step-by-Step Instructions

Step 1: Make and Chill the Pastry Dough

In a medium bowl, whisk together the all-purpose flour and fine sea salt. Add the cold, cubed butter. Using a pastry cutter or your fingertips, work the butter into the flour until the mixture looks like coarse crumbs with some pea-sized bits of butter still visible. Those small butter pockets are what make the crust flaky.

Sprinkle 3 tablespoons of ice water and the apple cider vinegar (if using) over the mixture. Gently toss with a fork until the dough just starts to come together. If it feels too dry or crumbly, add up to 1 more tablespoon of ice water, a teaspoon at a time. Do not overwork the dough; it should be a bit shaggy but hold when pressed.

Turn the dough onto a lightly floured surface, gather it into a ball, and gently press it into a flat disk about 1 inch thick. Wrap tightly in plastic wrap or parchment and refrigerate for at least 30 minutes (up to 2 days). Chilling allows the flour to hydrate and the butter to firm up, which helps with rolling and flakiness.

Step 2: Caramelize the Onions

While the dough chills, caramelize the onions. In a large skillet, heat the unsalted butter and olive oil over medium heat. Add the sliced onions and a pinch of salt. Stir to coat the onions in the fat, then reduce the heat to medium-low.

Cook the onions, stirring every few minutes, until they are very soft, deep golden brown, and jammy, about 25–30 minutes. If they start to brown too quickly or stick, lower the heat and add 1–2 tablespoons of water to deglaze the pan. The goal is slow, gentle caramelization, not charring.

When the onions are caramelized, stir in the minced garlic, thyme, black pepper, and red pepper flakes (if using). Cook for 1–2 minutes more, just until the garlic is fragrant. Taste and add a pinch more salt if needed. Remove from the heat and let cool slightly while you prepare the remaining components.

Step 3: Prep the Sweet Potatoes and Cheese

Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper if it is not already lined.

Peel the sweet potatoes and slice them into thin rounds, about 1/8 inch thick. For the most even slices, use a mandoline set to a thin setting, or a very sharp knife. Try to keep the slices similar in thickness so they cook evenly.

Place the sliced sweet potatoes in a bowl and sprinkle lightly with a pinch of salt and black pepper. Toss gently to distribute. Grate the Gruyère cheese (and Parmesan, if using) and set aside in a separate bowl. Having everything prepped and ready will make assembly quick and easy.

Step 4: Roll Out the Dough

Remove the chilled dough from the refrigerator and let it sit at room temperature for 5 minutes to soften just slightly, which helps with rolling. Place the dough on a large sheet of parchment paper that has been lightly dusted with flour.

Using a rolling pin, roll the dough into a rough 12-inch (30 cm) circle, about 1/8–1/4 inch thick. It does not need to be perfect; the charm of a galette is its rustic, free-form shape. If the dough cracks at the edges, simply patch and press it back together.

Slide the parchment with the rolled dough onto your baking sheet. This way you will not have to move the assembled galette later, which helps keep it intact.

Step 5: Assemble the Galette

Spread the caramelized onions evenly over the center of the dough, leaving a 2-inch (5 cm) border all around. This border will become the folded crust. Scatter about half of the grated Gruyère (and all of the Parmesan, if using) over the onions.

Arrange the sweet potato slices on top of the cheese in slightly overlapping concentric circles or shingled rows. Aim for a single, even layer so they cook through completely. If you have extra slices, tuck them into any gaps. Season the top lightly with another small pinch of salt and pepper.

Sprinkle the remaining Gruyère over the sweet potatoes. Do not pile the cheese too high on the exposed border, or it will leak as it melts.

Step 6: Fold, Bake, and Serve

Gently fold the uncovered edges of dough up and over the filling, working your way around the galette and pleating the dough as you go. You should still see the center of the filling; only the outer edge is covered by the crust.

In a small bowl, whisk together the egg and milk (or cream) to make an egg wash. Brush the exposed crust generously with the egg wash. This will give the crust a glossy, golden finish. Sprinkle the crust and visible sweet potatoes with a few extra thyme leaves and a pinch of flaky sea salt.

Bake the galette at 400°F (200°C) for 35–40 minutes, or until the crust is a deep golden brown, the cheese is melted and bubbling, and the sweet potatoes are tender when pierced with the tip of a knife. If the crust browns too quickly, loosely tent the edges with foil for the last 10 minutes of baking.

Remove the galette from the oven and let it rest on the baking sheet for at least 10 minutes to set. Slide it carefully onto a cutting board, garnish with a few more fresh thyme leaves, slice into wedges, and serve warm or at room temperature.

Pro Tips

• Slice the sweet potatoes evenly: Thin, uniform slices (about 1/8 inch) ensure the potatoes cook through completely in the same time the crust browns.
• Do not rush the onions: True caramelization takes a good 25–30 minutes over medium-low heat. Patience here adds a deep, sweet-savory flavor to the whole galette.
• Keep the dough cold: If it softens while rolling or assembling, pop the whole sheet pan into the fridge for 10 minutes before baking. Cold dough equals flaky crust.
• Flavor boost: A small pinch of flaky sea salt on top right out of the oven makes the sweet potatoes and cheese pop.
• Neat slices: Let the galette cool for at least 10 minutes before cutting to help the cheese set slightly and hold the wedges together.

Variations

• Goat cheese and rosemary: Swap half the Gruyère for crumbled goat cheese and replace thyme with finely chopped fresh rosemary for a tangier, more aromatic flavor.
• Smoky bacon twist: Cook 3–4 slices of chopped bacon until crisp, then mix into the caramelized onions before assembling for a smoky, savory version.
• Gluten-free option: Use your favorite gluten-free pie crust recipe or high-quality store-bought gluten-free pastry. Chill well before rolling to prevent cracking.

Storage & Make-Ahead

To store leftovers, let the galette cool completely, then cover tightly and refrigerate for up to 3 days. Reheat slices on a baking sheet in a 350°F (175°C) oven for 10–15 minutes, until warmed through and the crust has re-crisped. For make-ahead prep, you can:

Make the dough up to 2 days in advance and keep it refrigerated, or freeze it for up to 2 months (thaw overnight in the fridge before using). The caramelized onions can also be made 2–3 days ahead and refrigerated in an airtight container. Assemble the galette up to 4 hours before baking and keep it chilled, uncovered, in the refrigerator. Bake just before serving for the best texture.

Nutrition (per serving)

Approximate values per serving (1/6 of the galette): about 500–520 calories, 34 g fat, 41 g carbohydrates, 12 g protein, 3 g fiber, and 750–800 mg sodium (depending on how much salt you add). These numbers are estimates and will vary based on exact ingredient brands and portion sizes.