au gratin potatoes recipe

Ingredients: 2 pounds russet potatoes, peeled and thinly sliced 1 small onion, thinly sliced 2 cloves garlic, minced 1 1/2 cups heavy cream 1 1/2 cups whole milk 1/4 cup unsalted butter 2 tablespoons all-purpose flour 1 cup shredded Gruyère cheese 1 cup shredded sharp cheddar cheese