Heat the olive oil in a large pan over medium-high heat.
Brown the meatballs in batches until they are golden and caramelised on all over. They do not need to be fully cooked through at this stage.
Remove the browned meatballs to a large plate.
In the same pan which you used to brown the meatballs, sauté the onion and garlic with a generous pinch of salt for several minutes, or until the mixture has softened.
Add the tomato passata or canned tomatoes.
Add about 60 ml (1/4 cup) water to the bottle of tomato passata, swirl it around to "clean" the bottle, and pour this water into the pan.
Add the sugar.
Let the tomato sauce simmer gently for about 10 minutes.
Add the meatballs to the sauce, together with any of the meat juices which have collected on the plate.
Simmer the tomato sauce and meatballs for about 20-30 minutes on low heat, until the meatballs have cooked through completely (the timing will vary depending on the size of your meatballs) and the sauce has reduced and thickened slightly.
Taste for the sauce for seasoning.