In a frying pan over medium heat, warm oil and saute onions and garlic until caramelized; set aside. In a frypan over medium heat, cook beef mince until almost browned. Add onions and garlic then cook for 3 minutes.
In a saucepan over medium heat, combine tomato puree and tomatoes; bring to a boil then reduce heat. Simmer sauce for 15 minutes. Stir pepperoni, beef mince mixture and green capsicums into sauce; cover and simmer for 30 minutes.
Stir in mushrooms, garlic salt, salt, ground black pepper, onion powder, oregano and Italian seasoning. Simmer for 10 minutes and serve over your favourite pasta.