Holiday Leek and Potato Gratin With Gruyère

Quick Recipe Version (TL;DR)

• Yield: 6–8 servings
• Prep Time: 25 minutes
• Cook Time: 1 hour 10 minutes
• Total Time: 1 hour 45 minutes

Quick Ingredients

• 2 1/2 lb (1.1 kg) Yukon Gold potatoes, peeled and thinly sliced
• 3 large leeks, white and light green parts, sliced
• 3 tbsp unsalted butter
• 2 cloves garlic, minced
• 2 cups (480 ml) heavy cream
• 1 cup (240 ml) whole milk
• 2 1/2 cups (about 250 g) grated Gruyère cheese, divided
• 1/2 cup (45 g) finely grated Parmesan cheese (optional, for topping)
• 1 3/4 tsp kosher salt, divided
• 1/2 tsp freshly ground black pepper
• 1 tsp fresh thyme leaves (plus extra for garnish)
• Pinch ground nutmeg
• 1 tsp Dijon mustard (optional)
• Butter for greasing the baking dish

Do This

• 1. Preheat oven to 375°F (190°C). Generously butter a 3-quart (9×13-inch) baking dish.
• 2. Clean and slice leeks; sauté in 3 tbsp butter with 1/2 tsp salt until soft and lightly golden. Stir in garlic and thyme.
• 3. In a saucepan, gently warm cream, milk, remaining salt, pepper, nutmeg, and Dijon until steaming, not boiling.
• 4. Peel potatoes and slice very thin (about 1/8 inch / 3 mm). Pat dry with paper towels.
• 5. Layer half the potatoes in the dish, top with half the leeks and 1 cup Gruyère. Repeat with remaining potatoes, leeks, and 1 cup Gruyère.
• 6. Pour warm cream mixture evenly over the layers. Top with remaining 1/2 cup Gruyère and Parmesan.
• 7. Cover with foil and bake 40 minutes, then uncover and bake 30 minutes more until golden and bubbling. Rest 15 minutes, garnish with thyme, and serve.

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Why You’ll Love This Recipe

• It is a cozy, holiday-ready side dish that feels luxurious without being fussy.
• Buttery leeks, creamy potatoes, and nutty Gruyère create deep, layered flavor.
• Everything bakes in one dish, making it perfect for entertaining and potlucks.
• Easy to make ahead and reheat, so you are not tied to the stove during guests’ arrival.

Grocery List

• Produce: Yukon Gold potatoes, leeks, garlic, fresh thyme
• Dairy: Unsalted butter, heavy cream, whole milk, Gruyère cheese, Parmesan cheese (optional)
• Pantry: Kosher salt, black pepper, ground nutmeg, Dijon mustard (optional)

Full Ingredients

Main Gratin

• 2 1/2 lb (1.1 kg) Yukon Gold potatoes, peeled and sliced 1/8 inch (3 mm) thick
• 3 large leeks, white and light green parts only
• 3 tbsp unsalted butter
• 2 cloves garlic, minced
• 1 tsp fresh thyme leaves (plus extra for garnish)
• 1 3/4 tsp kosher salt, divided (use less if using fine salt)
• 1/2 tsp freshly ground black pepper
• Pinch ground nutmeg (about 1/8 tsp)
• 1 tsp Dijon mustard (optional but recommended)
• 2 cups (480 ml) heavy cream
• 1 cup (240 ml) whole milk
• 2 1/2 cups (about 250 g) coarsely grated Gruyère cheese, divided

Topping & Pan Prep

• 1/2 cup (45 g) finely grated Parmesan cheese (optional, for extra golden top)
• 1–2 tbsp unsalted butter, for greasing the baking dish
• Additional fresh thyme leaves or chopped chives, for garnish (optional)

Step-by-Step Instructions

Step 1: Preheat the oven and prepare your baking dish

Set a rack in the center of your oven and preheat to 375°F (190°C). Lightly soften 1–2 tablespoons of butter and use it to generously grease a 3-quart baking dish (a 9×13-inch rectangular dish or similar size gratin dish). Make sure to coat the corners and sides well so nothing sticks and the edges turn nicely golden and caramelized.

Gather all ingredients and have them measured out. This makes the layering process much smoother once the potatoes are sliced.

Step 2: Clean and slice the leeks

Leeks tend to hide grit between their layers, so take a moment to clean them thoroughly. Trim off the dark green tops and root ends, keeping only the white and light green parts. Slice each leek lengthwise in half, then cut into 1/4-inch (6 mm) half-moons.

Place the sliced leeks in a large bowl of cool water. Swish them around with your hands so any sand or dirt falls to the bottom. Lift the leeks out with your hands or a slotted spoon, leaving the grit behind, and drain well in a colander. Set aside.

Step 3: Sauté the leeks in butter

In a large skillet over medium heat, melt 3 tablespoons unsalted butter. Add the drained leeks and 1/2 teaspoon kosher salt. Cook, stirring often, for 8–10 minutes, until the leeks are very soft, fragrant, and just beginning to lightly caramelize around the edges.

Stir in the minced garlic and thyme leaves and cook for another 1 minute until the garlic loses its raw smell. Remove the pan from the heat and set the buttery leeks aside to cool slightly while you prepare the potatoes and cream mixture.

Step 4: Slice and dry the potatoes

Peel the potatoes. Using a sharp knife or a mandoline slicer, slice them into very thin rounds, about 1/8 inch (3 mm) thick. Try to keep the slices even so they cook at the same rate.

If you sliced the potatoes in advance, keep them in a bowl of cold water to prevent browning. Just before layering, drain and pat them very dry with clean kitchen towels or paper towels. Removing excess surface moisture helps the cream cling to the potatoes and prevents the sauce from becoming watery.

Step 5: Warm the cream mixture

In a medium saucepan, combine the 2 cups heavy cream, 1 cup whole milk, remaining 1 1/4 teaspoons kosher salt, 1/2 teaspoon black pepper, pinch of nutmeg, and Dijon mustard (if using). Place over medium-low heat and warm gently, stirring occasionally, until the mixture is steaming and small bubbles appear around the edges, about 4–5 minutes. Do not let it boil.

Warming the dairy helps it soak into the potatoes more quickly and shortens the baking time. Once steaming, remove from the heat and set aside.

Step 6: Layer the potatoes, leeks, and cheese

Spread about one-third of the potato slices in an even, slightly overlapping layer on the bottom of the buttered baking dish. Scatter half of the sautéed leeks evenly over the potatoes. Sprinkle with 1 cup of the grated Gruyère.

Add another one-third of the potatoes in an overlapping layer, followed by the remaining leeks and another 1 cup Gruyère. Finish with the remaining potato slices arranged neatly on top.

Slowly pour the warm cream mixture over the entire dish, pouring evenly so it seeps down between the layers. Gently press down on the potatoes with a spatula or clean hands to help them settle into the liquid.

Scatter the remaining 1/2 cup Gruyère and the 1/2 cup Parmesan (if using) evenly over the top for a deeply golden, bubbly crust.

Step 7: Bake until golden, then rest and serve

Cover the baking dish tightly with foil, tenting slightly so it does not stick to the cheese. Bake covered for 40 minutes. Then carefully remove the foil and continue baking, uncovered, for another 30 minutes until the top is deeply golden, the cream is bubbling around the edges, and a knife slides easily through the potatoes in the center.

Remove the dish from the oven and let it rest for at least 15 minutes before serving. This resting time allows the creamy sauce to thicken slightly and the layers to set, so each scoop holds together beautifully.

Just before serving, sprinkle with a few extra thyme leaves or finely chopped chives for a fresh, green contrast. Serve warm as a luxurious side dish alongside roasted meats, turkey, or a festive vegetarian main.

Pro Tips

• Slice evenly: Aim for consistent 1/8-inch (3 mm) slices so all the potatoes become tender at the same time. A mandoline makes this very easy.
• Do not skip drying the potatoes: Excess water dilutes the cream. Patting the slices dry gives you a richer, silkier sauce.
• Use the right cheese: A good-quality Gruyère melts smoothly and adds nutty depth. Avoid pre-shredded cheese if possible, as it often contains anti-caking agents that affect melting.
• Watch the top near the end: If the gratin is nicely browned but the potatoes still need a few more minutes, loosely tent with foil so the top does not burn.
• Let it rest: The 15-minute rest is crucial for clean slices and a creamy, not runny, texture.

Variations

• Smoky bacon version: Cook 4–6 slices of diced bacon or pancetta until crisp, then remove and sauté the leeks in the rendered fat (reduce the butter). Sprinkle the crispy bits between the layers for a smoky, savory twist.
• Cheese swap: Replace some or all of the Gruyère with Comté, Emmental, or sharp white cheddar for a different flavor profile while keeping the same method.
• Herb and garlic boost: Add extra thyme or a mix of thyme and rosemary, and roast a whole head of garlic to mash into the cream for a deeper, sweet-garlic note.

Storage & Make-Ahead

To store leftovers, let the gratin cool completely, then cover the dish tightly or transfer portions to airtight containers. Refrigerate for up to 4 days. Reheat, covered with foil, in a 350°F (175°C) oven for about 20–25 minutes, or until hot and bubbling.

For make-ahead, you can assemble the entire dish (through layering and adding cream) up to 24 hours in advance. Cover tightly and refrigerate. When ready to bake, let it sit at room temperature for 20–30 minutes while the oven preheats, then bake as directed, adding 10–15 extra minutes if needed.

You can also bake the gratin fully, cool, and refrigerate. Reheat covered at 350°F (175°C) for 25–30 minutes. Freezing is possible: wrap tightly and freeze for up to 2 months. Thaw overnight in the refrigerator before reheating, and add a splash of cream if it looks dry.

Nutrition (per serving)

Approximate values per serving (based on 8 servings, using Parmesan): about 530 calories, 36 g fat (of which 22 g saturated), 35 g carbohydrates, 3 g fiber, 7 g sugar, 17 g protein, and around 620 mg sodium. Values will vary based on exact ingredients and portion sizes.