To make the potatoes boil the water, butter and onions until boiling. Once boiling turn the heat off and add the milk, instant potatoes, and salt and pepper, mix well. Then add the mustard to the potatoes.
Cut up the smoked sausage and add to the bottom of a 13x9 baking dish. (If you want to make this dish a little less greasy you can cut up the sausage then brown it on both sides in a pan and drain it.)
Put the potatoes on top of the sausage and smooth out. Put the cheese on top of the potatoes.
Bake on 350F for 25-30 minutes or until cheese is melted and just starting to brown. Let stand for 5-10 minutes. I usually serve this with green beans. Its ohhhh so yummy lol.