In a large nonstick skillet, cook ham until browned, 4-5 minutes; drain and pat dry. In a greased 2-qt. baking dish, layer the turkey, cheese and ham; set aside. , In a large saucepan, saute onion in butter until tender. Stir in the flour, mustard and nutmeg until blended. Gradually stir in milk. Bring to a boil; cook and stir until thickened, about 2 minutes. Pour over ham. , Combine topping ingredients; sprinkle over the top. Bake, uncovered, at 350° until golden brown and bubbly, 25-30 minutes.