Slice potatoes and place on a good size sheet of aluminum wrap (enough to cover all ingredients). Add mushrooms and green pepper and onion (in that order), placing dabs of butter on top of raw ingredients. Add fried Italian sausage on top of all and then seal aluminum foil tightly with a double fold to keep from outside air. Place on a light grill for one hour. The foil will bulge out like a pillow as the ingredients cook into each other. The smell will be very tempting but don't open for a full hour. Remove this from grill and open foil package. The butter will cook into the potatoes putting a slight burn on them (which enhances the flavor). Add Velveeta to the hot dish if desired and give 10 minutes to melt.