1 cup diced fully cooked smoked extra-lean ham (about 6 ounces)
1/4 cup finely chopped onion
Heat oven to 350 degrees. Spray 1 1/2 quart casserole with nonstick cooking spray. Heat margarine in 1 /12 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 the ham, half the onion and one-third of the sauce in the casserole; repeat. Top with remaining potatoes and sauce. Cover and bake 30 minutes. Uncover and bake about 40 minutes or until potatoes are tender. Let stand 5 to 10 minutes before serving.