Heat 1 tbsp olive oil in a large, non-stick frying pan over a medium-low heat and cook the onion for 6-7 minutes, until softened, adding the garlic for the final minute. Place half the cooked onion and garlic in a large bowl, and set the other half aside.
Add the beef mince and half the chopped parsley to the large bowl and season. Divide the beef mixture into 12.
Divide the mozzarella into 12 pieces and wrap the beef mince around it to enclose it completely and form a round meatball. Repeat with the remaining beef mince and mozzarella.
Heat the remaining 1 tbsp olive oil in the frying pan over a medium-high heat and cook the meatballs for 5 minutes, turning frequently, until nicely browned.
Reduce the heat to medium-low and add the passata and the chilli to the pan. Season with freshly ground black pepper and simmer the meatballs in the sauce for 10 minutes, until the sauce has reduced slightly.
While the meatballs are simmering, cook the spaghetti according to the instructions on the packet. Divide the spaghetti between 4 bowls and spoon over the meatballs. Scatter over the remaining parsley to serve.