Cook shells in salt water with a drop of oil. Drain water and fill with fresh water otherwise shells will dry out. Set aside. Saute onion and garlic. Dice pepper and mushrooms and saute for additional 15-20 minutes. Add spices.
Fill each shell wiht vegetables and cheese and top with seasoned tomato sauce. Bake in the oven at 400 for about five minutes or until cheese is fullly melted.