Cook pasta according to package directions. Meanwhile, in a large skillet, cook beef and onions over medium heat until meat is no longer pink; drain. Add the spaghetti sauce, tomatoes, mushrooms, garlic, salt and pepper. Bring to a boil. Reduce heat; simmer, uncovered, for 20 minutes, stirring occasionally.
Drain pasta; place half in a greased 3-qt. baking dish. Top with half of the beef mixture. Layer with sour cream and provolone cheese. Top with remaining pasta and beef mixture. Sprinkle with mozzarella cheese.
Cover and bake at 350° for 40 minutes. Uncover; bake 5-10 minutes longer or until cheese is melted. Yield: 6-8 servings.