2 cloves garlic, minced
2 medium onions, finely chopped
4 tablespoons unsalted butter
1 ½ tablespoon flour
1 cup milk
1 teaspoon dry mustard
Salt and pepper
¼ cup grated Pecorino Romano
¼ cup asiago cheese
¼ cup gruyere cheese
2lbs russet potatoes, peeled and very thinly sliced
Preheat the oven to 375F.
Heat the butter in a medium pan over a medium-high heat. Once fully melted, add the onion and garlic and sauté until transluscent.
Add the flour and cook 2-3 minutes.
Add the milk and dry mustard and bring to a simmer. Cook until the mixture begins to thicken, about 5 minutes.
Remove the mixture from the heat and add the cheese. Stir until the cheese has melted and the mixture is homogenous.
Lay the potato slices out in a thin, even layer in a 13x9x2 inch baking dish. Pour a layer of the cheese mixture on top. Repeat, ending with the cheese mixture.
Cover the casserole with aluminum foil and press down firmly and evenly to ensure there are no air bubbles.
Bake for 1 hour, then remove the foil and bake an additional 15-20 minutes, or until brown and crispy.
Allow casserole to cool for at least 10 minutes before serving.