In a large, nonstick skillet, sauté ham for 4-5 minutes or until browned; drain and pat dry.
Meanwhile, In a greased 2 qt. baking dish, layer the turkey, cheese and ham, set aside.
In a large saucepan, sauté the onion in butter until tender, stir in the flour, mustard and nutmeg until blended. Gradually stir in the milk. Bring to a boil, cook and stir for 2 minutes or until thickened. Pour over ham.
Combine topping ingredients. Sprinkle over the top. Bake, uncovered, at 350 for 25-30 minutes or until golden brown and bubbly. Serves 6