Heat the slow cooker if necessary. Heat the oil in a non-stick frying pan and add the onion, carrots, celery and garlic and fry gently for a minute. Pour in the passata, add the parsley and stir, then transfer the lot to the slow cooker.
To make the meatballs, tip the mince into a large bowl. Add the oats, paprika, garlic and plenty of black pepper, and mix everything together with your hands. Divide the mixture into 20 lumps about the size of a walnut and roll each piece into a meatball. Spray or run a non-stick pan with a little oil and gently cook the meatballs until they start to brown. Add them to the tomato base and cook on Low for 5 hours. Serve over rice or pasta if you like, or with a green salad.