In a Dutch oven, brown beef and onion over medium heat until meat is no longer pink; crumble beef; drain. Add the spaghetti sauce, mushrooms, salt, garlic powder, oregano and pepper; heat through., Spoon half the mixture into a 5-qt. slow cooker. Top with half the noodles, pepperoni and cheeses. Repeat layers. Cover and cook on low for 4-5 hours or until cheese is melted.