Cook pasta according to package directions. Meanwhile in a large saucepan, cook the beef, onions and garlic over med. heat until meat is no longer pink; drain. Stir in spaghetti sauce and mushrooms; bring to a boil reduce heat; cover and simmer for 20 min.
Place 1/2 cup of meat sauce in a greased shallow 2 1/2 qt. baking dish. Drain pasta; place half over sauce top with half of the remaining meat sauce; spread with sour cream. Top with process cheese and remaining pasta and meat sauce. Sprinkle with mozzarella cheese.
Cover and bake at 350 degrees F for 25-30 minutes. Uncover; bake 5-10 minutes longer or until bubbly.