Cook roast in crock pot with 1 cup water all day (6 to 10 hours depending on temp.) save liquid to make gravy. The meat should be falling apart. Separate the meat until it is entirely shredded.
Sauté onion, carrots, and celery, for 10 to 15 minutes or until vegetables are wilted, add garlic powder, salt, pepper, and nutmeg.
Stir in the gravy. Pour into large shallow baking pan and cool. The filling should be about 1 1/2 inches deep.
Cover the meat mixture in the pan with the corn and then top with the hot mashed potatoes. Smooth potatoes evenly, brush surface with butter.
Bake at 325°F for 35 to 40 minutes.