Cook pasta according to package directions.
In a large skillet brown the ground beef with the onions and garlic.
Remove from heat and stir in spaghetti sauce and mushrooms.
Place 1/2 cup of meat sauce in a greased shallow 2 1/2 quart baking dish.
Place half of pasta over meat sauce.
Place 1/2 of the remaining meat sauce.
Cover with sour cream
Top with American Cheese.
Cover with remaining pasta and meat sauce.
Sprinkle with Mozzarella cheese.
Cover and bake at 350° for 25-30 minutes.
Uncover and bake 5-10 minutes longer until bubbly.