Pour the tomato sauce all over the meatballs and onions, turn the oven down to 200C/180C Fan/Gas 6 and bake for 35 minutes more. Check the meatballs are cooked through and the sauce has thickened.
(Or you can chop as finely as humanly possible by hand.) Reserve 2 tablespoons of the mixture for the meatballs. Warm the garlic oil in a large, heavy-based saucepan or casserole, add the onion ...