Place ciabatta in a bowl with milk; let stand for 5 minutes.
Heat 2 tablespoons oil in a non-stick frying pan over medium heat, add half the onion and half the garlic, and cook for 2-3 minutes. Finely chop 3 slices of the pancetta; add to the pan with anchovy and rosemary and cook for 1-2 minutes.
Place both mince in a large bowl. Squeeze excess milk from bread and add to mince mixture along with onion mixture and parmesan. Season with sea salt and freshly ground black pepper and mix until well combined. Divide into four and roll into large patties.
Wrap each meatball in three slices of pancetta and refrigerate for 10 minutes. Heat 2 tablespoons olive oil in a large, deep frying pan over medium heat and cook meatballs for 3-4 minutes on each side, or until golden. Drain on paper towel and set aside.
Add remaining onion and garlic to the pan and cook for 2-3 minutes, or until soft. Add chilli, passata, and stock and cook, stirring, for 7-8 minutes, or until mixture thickens. Add remaining olive oil and stir to combine. Add meatballs back to the pan with basil leaves.
Reduce to a simmer and cook, covered, for 10-15 minutes, or until cooked through.