Place the minced beef and pork in a bowl with the egg, parsley and breadcrumbs. Season with salt and pepper and using your hands give everything a good mix.
Using slightly damp hands, make about 18-20 even-sized balls. Arrange on a flat baking sheet, drizzle over a little olive oil to lightly coat and chill for an hour if time allows.
Heat half the olive oil in a large pan and sauté the onion and celery for about 5 minutes until lightly golden. Stir in the garlic and chilli, if using, and cook for another minute, stirring. Add the tomato purée and passata and simmer for a few minutes until combined. Season with salt and pepper.
Heat the rest of the olive oil in a deep-sided frying pan and cook the meatballs for 5-6 minutes until almost cooked through and nicely browned. Pour in the sauce and simmer gently for another 8-10 minutes until reduced and thickened.
Meanwhile, plunge the spaghetti into a large pan of boiling salted water and simmer for 8-10 minutes until tender but still with a little bite, as the Italians say ‘al dente’. Drain well and return to the pan and then add a few ladlefuls of the tomato sauce to coat.
Divide among pasta bowls and spoon the meatballs with the rest of the tomato sauce on top. Scatter over the cheese and a little chopped parsley.
Serve with a big bowl of green salad.