Cook pasta according to package directions. Meanwhile, in a large saucepan, cook the beef, onions and garlic over medium 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 minutes.
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° for 25-30 minutes. Uncover; bake 5-10 minutes longer or until bubbly.
