Heat butter, add flour, 1 teaspoon salt and pepper and let bubble.
Stir in milk and cook and stir until mixture comes to a boil and is thickened.
Combine potatoes, onion, 1 teaspoon salt in saucepan.
Add boiling water an inch deep.
Cover and simmer 10 minutes.
Spread half the potatoes in buttered 1 1/2 quart casserole.
Add half the sauce, sprinkle with half the parsley.
Bake in a 350° F.oven 45 minutes or until tender and browned.