Warm up a large pot of lightly salted water. Cook for 8 to 10 minutes, or until pasta is al dente; drain and place in a large saucepan.

Meanwhile, brown the beef in a large skillet over medium heat; just before the beef is browned, stir in the onion.

Stir in tomato sauce, mushrooms, garlic, salt, pepper, and stewed tomatoes to beef mixture. Cook for 10 to 15 minutes on low heat, or until thoroughly heated.