Place acorn squash in pre-heated oven at 350 F. Placed in casserole pan filled w/ 2 inches of water. Cover top w/ foil.
Bake for an hour then remove and slice in half, removing seeds in center. Return to pan and bake another hour.
Remove and slice small amount off bottom so to sit level on plate. Saute chopped onions, bell peppers, and sliced mushrooms. Fill w/ cooked rice, mushrooms, onions and bell peppers. Serve as a greatly filling and nutritious vegetarian dish.