In a large saucepan, heat the oil and cook the onion for 5 minutes until softened. Remove half of the onion and reserve for later. Add the garlic, carrots and celery to the pan and cook for a further 5 minutes. Pour in the chopped tomatoes, beef stock, Worcestershire sauce, tomato purée and dried mixed herbs. Stir until combined then simmer for 30 minutes.
Meanwhile, make the meatballs. Preheat the grill to medium-high. In a bowl, mix together the reserved cooked onion, mince, breadcrumbs, egg and half of the parsley. Using clean hands, shape into 12 mini meatballs and put on a lined baking tray. Grill for 10-15 minutes, turning halfway, until the meatballs are cooked through with no pink remaining. Stir the cooked meatballs through the tomato sauce along with the rest of the parsley.
Serve the meatballs and sauce in the white finger buns, with a sprinkling of extra parsley leaves if you have any left.
Cook’s tip: if you are planning a picnic, spoon the meatballs and the sauce carefully into a wide-neck flask (you may need two flasks), and tuck in when you get to your picnic spot.