scalloped potato recipe

Ingredients: 4 large russet potatoes, peeled and thinly sliced 1 medium onion, thinly sliced 3 tablespoons unsalted butter 3 tablespoons all-purpose flour 2 cups milk 1 cup heavy cream 2 cups shredded cheddar cheese Salt and pepper to taste 1/2 teaspoon garlic powder (optional)