In a frying pan, brown the beef, onion, and garlic on medium-high until no pink remains; drain any fat. Add beef mixture to a 6QT slow cooker.
Add remaining ingredients, except for spinach and cooked pasta. Cover and cook on HIGH for 3-4 hours or LOW for 7-8 hours or until mushrooms and peppers are tender.
In a separate pot, cook the pasta al dente following the package directions.
Mix in the pasta and spinach. Cover and cook for 10-15 minutes more. Ladle into individual bowls and add desired toppings.
