Heat the oil in the oven 200oC and add the onion, cook for 4-5 mins over a low heat, then stir in the garlic. Leave to cool completely. To make the tomato sauce, heat the oil in the oven at 200oC and fry the onion, carrot and celery for 3-4 mins, then add the garlic and cook for a minute. Add the passata and bring to a simmer, then season and cook for 5 mins. Select the oven settings as above. Mix the cooled onion with the tomato purée, mince, egg and breadcrumbs, and season. Line a 900g loaf tin with a liner or baking parchment and pack in the meatloaf mixture. Cook on the above program. Leave to rest for 10 mins before lifting out of the tin. Serve slices of the meatloaf with some tomato sauce spooned over.