Cheddar Au Gratin Potatoes Recipe

Preheat and prepare potatoes

Preheat oven to 375°F (190°C). Lightly butter a 9x9-inch or similar sized shallow baking dish with 1 tablespoon unsalted butter. Peel potatoes if desired (Yukon Gold can be left partially peeled for texture). Using a mandoline or sharp knife, slice potatoes 1/8- to 3/16-inch thick and place in a large bowl of cold water to remove excess starch.

Drain the sliced potatoes and pat dry thoroughly with clean kitchen towels or paper towels. Toss gently with 1 teaspoon kosher salt and 1/2 teaspoon freshly ground black pepper; set aside.

Trim leeks to use only white and light green parts. Slice thinly and rinse in a bowl of cold water, lifting the leeks out to drain to remove grit. Dry briefly.

In a 10-inch skillet over medium heat, melt 2 tablespoons unsalted butter. Add the sliced leeks and cook, stirring occasionally, until softened and translucent, about 5–7 minutes. Add 1 minced garlic clove in the last 1 minute and cook until fragrant. Remove from heat and let cool slightly.

In a small bowl whisk together 1 tablespoon flour with a few tablespoons of the cold milk to make a smooth slurry. Set aside.

In a saucepan over medium heat, combine 1 1/2 cups heavy cream and 1/2 cup whole milk. Warm gently until small bubbles form at the edges but do not boil. Whisk in the flour slurry and continue to cook, whisking, until the mixture thickens slightly, about 2–3 minutes.

Stir in 1 teaspoon Dijon mustard and 1/8 teaspoon freshly grated nutmeg. Remove the pan from heat and gradually stir in 2 cups freshly grated sharp cheddar cheese until melted and smooth. Taste and adjust seasoning with more salt and pepper if needed. Reserve about 1/4 cup cheddar mixed with the Parmesan for topping.

Arrange one-third of the potato slices in an even layer in the prepared baking dish, slightly overlapping. Sprinkle about one-third of the cooked leeks over the potatoes and spoon about one-third of the cheddar cream sauce over the layer, spreading gently. Repeat with a second layer, then finish with the remaining potato slices. Pour any remaining sauce over the top, ensuring even coverage.

Sprinkle the reserved 1/4 cup cheddar and 1/4 cup grated Parmesan evenly over the top (or just the cheddar if not using Parmesan). Cover the dish tightly with aluminum foil.

Bake covered for 30 minutes at 375°F (190°C). Remove foil and continue to bake until the potatoes are tender when pierced with a knife and the top is golden and bubbling, about 20–25 more minutes. If the top browns too quickly, tent loosely with foil.

Let the gratin rest for 10 minutes before serving to set the sauce. Garnish with snipped fresh chives if desired.