Scalloped Potato Skillet Gratin with Gruyère, Leeks, and Black Pepper

1. Prepare the leeks

Preheat your oven to 350°F. In a 12-inch oven-safe skillet, melt 2 tablespoons of unsalted butter over medium heat. Add the sliced leeks to the skillet, seasoning them with 1 teaspoon of kosher salt, ¾ teaspoon of freshly ground black pepper, and 2 tablespoons of chopped fresh sage. Cook the leeks for 7 to 10 minutes, stirring occasionally, until they're soft and fragrant.

2. Slice the potatoes

While the leeks are cooking, peel and slice the Yukon Gold potatoes using a mandoline or a sharp knife. Aim for thin, even slices to ensure they cook uniformly in the gratin.

3. Blend the milk mixture

Once the leeks are ready, add 1¼ cups crème fraîche, ½ cup whole milk, 2 finely chopped garlic cloves, and 2 bay leaves to the skillet. Bring the mixture to a gentle simmer, allowing the garlic to infuse the milk with its savory depth. After about 5 minutes, remove the bay leaves to prevent their flavor from becoming overpowering.

4. Prepare the egg mixture

In a large heatproof bowl, whisk together 4 large eggs with ¼ teaspoon of freshly grated nutmeg. Slowly temper the eggs by gradually whisking in the hot milk mixture from the skillet, ensuring the eggs don’t scramble.

5. Assemble the gratin

Grease the skillet with butter, then arrange half of the sliced potatoes in an even layer. Season the potatoes lightly with salt and pepper. Spread half of the cooked leeks over the potatoes, then pour over half of the egg mixture. Layer the remaining potatoes on top, season again, and add the rest of the leeks. Pour the remaining egg mixture over the top, ensuring everything is well-covered, and finish by sprinkling 1¼ cups of grated Gruyère cheese over the surface.

6. Bake the gratin

Cover the skillet with aluminum foil and place it in the preheated oven. Bake for 40 minutes, allowing the potatoes to cook through as the steam is trapped under the foil. After 40 minutes, remove the foil and continue baking for an additional 20 to 25 minutes. During this time, the cheese will melt into a bubbling, golden crust that adds a delightful texture and flavor contrast. Once the top is golden brown and the cheese is nicely crisp, your gratin is ready.