Cut squash in half lengthwise; scoop out seeds. Place squash, cut side down, in a baking dish. Fill pan with hot water to a depth of 1/2 inches Bake, uncovered, at 375° for 30-40 minutes or until tender.
When cool enough to handle, scoop out squash, separating strands with a fork; set shells and squash aside.
In a skillet, cook beef, onion and green pepper over medium heat until meat is no longer pink and vegetables are tender; drain.
Add mushrooms, garlic, basil, oregano, salt and pepper; cook and stir for 2 minutes. Add tomatoes; cook and stir for 2 minutes. Add squash; mix well.
Cook, uncovered, until liquid has evaporated, about 10 minutes. Fill shells; place in shallow baking dish.
Bake, uncovered, at 350° for 15 minutes. Sprinkle with cheese; bake 5 minutes longer or until cheese is melted.