Pour milk into 2 cups glass measure. Cook in microwave 1 minute 30 seconds, set aside
Place DF butter in 2 cups glass measure. Cook 45 seconds. Stir in flour and cook 45 seconds more.
Briskly stir hot milk, mustard and salt and pepper into butter/flour mixture. (Mixture will thicken with cooking)
Place potatoes, ham, and onion in 1 1/2 quart casserole.
Pour in milk mixture and stir to combine.
Cover and cook 18 minutes (Cook time will vary depending microwave), stirring twice during cooking time. (You can also bake in a 350 degree oven for about 30 minutes if you prefer)
Remove from microwave/oven. If using cheese, sprinkle in on top and cover the casserole to allow the cheese to melt, about 5 minutes.