3 pounds Yukon Gold potatoes, peeled and sliced into 1/8-inch rounds
3 cups whole milk
4 tablespoons unsalted butter
1/4 cup all-purpose flour
2 cups sharp cheddar cheese, shredded
1 cup Gruyere cheese, shredded
3 cloves garlic, minced
1/2 small yellow onion, finely diced
1 teaspoon sea salt
1/2 teaspoon black pepper
1/4 teaspoon ground nutmeg
1 tablespoon fresh thyme leaves
1/2 teaspoon smoked paprika for the topping
Instructions
Preheat your oven to 375 degrees Fahrenheit and grease a 9×13 inch baking dish generously with butter. Peel your potatoes and slice them into thin, even rounds. Using a mandoline slicer is highly recommended to ensure uniform thickness, which helps the potatoes cook evenly.
In a large saucepan or skillet, melt the butter over medium heat. Add the finely diced onion and cook for about 3 to 4 minutes until it becomes translucent. Stir in the minced garlic and cook for another minute until fragrant, being careful not to let the garlic burn.
Sprinkle the flour over the butter and onion mixture. Whisk constantly for about 2 minutes to cook out the raw flour taste. Slowly pour in the milk, whisking continuously to prevent lumps from forming. Continue to cook and stir until the sauce thickens enough to coat the back of a spoon.
Remove the saucepan from the heat. Stir in the salt, black pepper, nutmeg, and fresh thyme. Add 1.5 cups of the cheddar cheese and 1/2 cup of the Gruyere cheese. Stir until the cheese is completely melted and the sauce is smooth and glossy.
Arrange half of the potato slices in an even layer in the prepared baking dish. Pour half of the cheese sauce over the potatoes. Repeat with the remaining potatoes and the rest of the sauce. Use a spatula to spread the sauce evenly so every potato slice is covered.
Cover the dish tightly with aluminum foil and bake for 45 minutes. Remove the foil and sprinkle the remaining cheese and a dash of smoked paprika over the top. Return the dish to the oven and bake for another 25 to 30 minutes, or until the potatoes are tender and the top is bubbly and golden brown.
