Place the minced beef and pork in a bowl with the breadcrumbs, parsley and egg. Season with salt and pepper and using your hands give everything a good mix. Using slightly wetted hands, make about 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 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 passata and tomato puree and simmer for a few minutes until nicely combined. Season with salt and pepper.
Add the meatballs to the sauce and simmer gently for another 10-15 minutes until the meatballs are cooked through and the sauce has nicely reduced, turning the meatballs occasionally with a tongs to ensure they cook evenly.
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, if liked to serve.