Preheat the oven to 375°F (190°C). Gather all ingredients and equipment.

In a medium saucepan, melt 2 tablespoons of butter. Whisk in 2 tablespoons of all-purpose flour and cook until bubbly.

Gradually add 2 cups of milk to the roux, whisking continuously until thickened. Stir in sharp cheddar and Gruyère until melted.

In a greased baking dish, layer half of the sliced potatoes and half of the cheese sauce. Repeat with remaining potatoes and sauce.

Sprinkle ½ cup of breadcrumbs over the final cheese sauce layer.

Cover with foil and bake for 45 minutes. Remove foil and bake an additional 15-20 minutes until golden brown.

Let the dish rest for 10 minutes before serving.