In a small saucepan stir together milk, flour, salt, pepper, and garlic powder, if desired. Cook and stir over medium heat until thickened and bubbly. Stir in onion.
Spray a 1-1/2-quart casserole with nonstick spray coating. Place half of the potatoes in the casserole. Top with half of the sauce. Repeat layers with remaining potatoes and sauce.
Bake, covered, in a 350 degree F oven about 65 minutes or until potatoes are tender, stirring once. Remove from oven and sprinkle with cheese. Cover and let stand 1 to 2 minutes or until cheese is melted. If desired, garnish with parsley. Makes 8 servings.