Heat oven to 350º. Spray 1 1/2-quart casserole with nonstick cooking spray.
Melt margarine in 1 1/2-quart saucepan over medium heat. Stir in flour, mustard, salt and pepper. Cook, stirring constantly, until margarine is absorbed. Gradually stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute.
Layer one-third of the potatoes, half of the ham, half of the onion and one-third of the sauce in casserole; repeat. Top with remaining potatoes and sauce.
Cover and bake 1 hour or until potatoes are tender. Let stand 5 to 10 minutes before serving. You may add 1/2 cup of shredded cheddar cheese to the last 10 minutes of baking for added flavor.