Heat olive oil in a large saucepan over medium heat
Add chopped onion and cook until soft
Add minced garlic and cook briefly
Add ground beef or Italian sausage if desired
Cook until browned, then drain excess fat
Stir in tomato paste and cook for 1 minute
Add crushed tomatoes and tomato sauce
Add salt, black pepper, dried oregano, dried basil, and red pepper flakes if desired
Add a pinch of sugar if needed
Stir well and bring to a simmer
Reduce heat to low and cook for 20 to 45 minutes
Stir occasionally
Add fresh basil or parsley near the end if desired
Taste and adjust seasoning
Serve over cooked spaghetti