Wash zucchini. Trim off ends of zucchini. Cut zucchini in half lengthwise. Scoop out pulp leaving 1/2-inch shells, finely chop pulp. In a skillet, cook meat, zucchini pulp, onion (or peppers, mushrooms if using) over medium heat. Drain when meat is fully cooked. Remove from heat. Cook rice or mashed potatoes. Remove from heat when done. Combine skillet mixture with rice or potatoes. Spoon it all into the zucchini shells. Place in a greased baking dish. Sprinkle with cheese, salt and pepper. Bake uncovered at 350 degrees for about 30 minutes or until zucchini is tender.