Sweet Potato and Caramelized Leek Gratin with Gruyère

Why You’ll Love This Recipe

• Silky layers of sweet potato meet sweet, jammy leeks for a cozy, elegant side.
• Gruyère melts into a creamy sauce with a crackly golden top that shatters beautifully.
• Gentle thyme and a whisper of nutmeg add depth without overpowering.
• Make-ahead friendly and perfect for holidays or a special Sunday dinner.

Grocery List

• Produce: Sweet potatoes, leeks, garlic, fresh thyme
• Dairy: Heavy cream, whole milk, Gruyère cheese, unsalted butter
• Pantry: Olive oil, kosher salt, black pepper, whole nutmeg (or ground)

Full Ingredients

For the Gratin

• 2 lb (900 g) sweet potatoes, peeled and sliced 1/8 inch (3 mm) thick
• 6 oz (170 g) Gruyère cheese, shredded, divided (about 1 1/2 cups)
• 1 tbsp unsalted butter, softened (for greasing the dish)

Caramelized Leeks

• 2 large leeks (white and light green parts only), cleaned and thinly sliced (about 3 cups)
• 2 tbsp unsalted butter
• 1 tbsp olive oil
• 2 garlic cloves, minced
• 1 tbsp fresh thyme leaves, chopped
• 1/2 tsp kosher salt (for leeks)

Cream Mixture

• 1 1/2 cups (360 ml) heavy cream
• 1/2 cup (120 ml) whole milk
• 1/8 tsp freshly grated nutmeg
• 3/4 tsp kosher salt
• 1/2 tsp freshly ground black pepper

Step-by-Step Instructions

Step 1: Preheat and prepare the dish

Preheat the oven to 375°F (190°C). Generously butter a 2-quart (2 L) shallow gratin dish or 9-inch square baking dish with the softened butter. This helps the first layer release and promotes browning on the edges.

Step 2: Clean and slice the leeks

Trim away the dark green tops and the root ends. Split leeks lengthwise and fan the layers under cool water to remove grit. Pat dry. Slice into thin half-moons. Properly cleaned leeks are essential for a grit-free gratin.

Step 3: Caramelize the leeks with thyme and garlic

Heat 2 tbsp butter and 1 tbsp olive oil in a large skillet over medium heat. Add the leeks and 1/2 tsp kosher salt. Cook, stirring occasionally, until soft, sweet, and lightly golden at the edges, 12–15 minutes. Lower the heat if browning too quickly. Stir in the minced garlic and chopped thyme and cook 1 minute more until fragrant. Remove from the heat.

Step 4: Warm the cream with nutmeg

In a small saucepan, combine the heavy cream, milk, nutmeg, 3/4 tsp kosher salt, and black pepper. Heat over medium-low until steaming and tiny bubbles appear at the edges, 3–5 minutes. Do not boil. Warming the cream helps it permeate the layers evenly and reduces curdling risk.

Step 5: Slice the sweet potatoes evenly

Peel and slice sweet potatoes into 1/8-inch (3 mm) rounds using a sharp knife or mandoline. Consistent thickness ensures even cooking and clean layers. If prepping ahead, keep slices loosely covered to prevent drying.

Step 6: Layer the gratin

Arrange a slightly overlapping layer of sweet potato slices in the prepared dish. Scatter one-third of the caramelized leeks over the potatoes and sprinkle with about one-third of the Gruyère. Repeat to create 3 total potato layers and 2 leeks-and-cheese layers, finishing with a final potato layer. Gently pour the warm cream mixture evenly over the entire dish, tilting the dish so it seeps between layers. Top with the remaining Gruyère.

Step 7: Bake until tender and crackly golden

Cover the dish tightly with foil and bake for 35 minutes. Uncover and continue baking until the potatoes are completely tender, the cream is bubbling at the edges, and the top is deeply golden and crackly, 15–20 minutes more. For extra color, broil for 1–2 minutes, watching closely. Let the gratin rest 10 minutes before serving to set the layers. Garnish with a few fresh thyme leaves if you like.

Pro Tips

• Rinse leeks thoroughly. Grit hides between layers; split them lengthwise to wash.
• Use a mandoline for even 1/8-inch slices, which cook through at the same rate.
• Warm the cream first. Cold dairy can curdle and won’t penetrate the layers as well.
• Bake covered, then uncovered. This keeps the interior tender and the top crisp.
• Rest before slicing. Ten minutes lets the creamy sauce thicken and layers hold together.

Variations

• Smoky bacon: Crisp 4 oz diced bacon, drain, and fold into the leeks before layering.
• Cheese swap: Try Comté or Emmental for a similar melt and nutty flavor profile.
• Herb twist: Replace thyme with rosemary and a hint of sage for a woodsy, wintry vibe.

Storage & Make-Ahead

Assemble up to 24 hours ahead: layer the dish, pour in the warm cream, cover tightly, and refrigerate. Bake straight from the fridge, adding 10–15 extra minutes before uncovering. Leftovers keep 3–4 days in the refrigerator; reheat covered at 350°F (175°C) for 15–20 minutes, then uncover for 5 minutes to re-crisp. For best texture, avoid freezing after baking; if freezing, freeze the assembled but unbaked gratin for up to 2 months, thaw overnight in the fridge, then bake as directed.

Nutrition (per serving)

Approximate: 520–560 calories; 36–42 g fat; 32–38 g carbohydrates; 12–16 g protein; 520–650 mg sodium; 4–6 g fiber. Values will vary based on brands and exact measurements.