While the oven baked meatballs cook, mix remaining onion and the garlic in a small bowl. Return pan to oven with onion mixture until softened, about 5 minutes.
Add tomato passata or crushed tomatoes and water to the pan, then mix to combine everything.
Continue cooking your oven baked meatballs with tomato sauce for an additional 30 minutes. Add more water if the sauce becomes too thick.
Serve the oven baked meatballs and tomato sauce with cooked spaghetti; top with extra parmesan cheese and basil leaves.