Peel the potatoes and cut into 1/4-inch slices. Boil the potatoes in salted water until almost tender; drain.
For the sauce, melt butter in a medium saucepan over medium heat. Add flour and cook until the flour is dissolved and the sauce is smooth and bubbly, stirring constantly. Add the half-and-half gradually, stirring constantly. Increase the heat to medium-high. Cook until thickened, stirring constantly. Add the onion, dried parsley, garlic salt and pepper. Stir in the cheese.
Combine the potatoes and sauce in a large bowl an d mix gently. Pour into a greased 9x13-inch baking dish. Top with the cheese. Mix the bread crumbs with the butter in a small bowl. Sprinkle over the potato mixture. Sprinkle with the parsley flakes. Bake at 350 F for 25 minutes or until bubbly.