For the sauce:
1) In a large heavy frying pan, heat the oil over medium heat. Add the pancetta and cook, stirring constantly, until golden brown, about 5 to 7 mins. Using a slotted spoon, remove the pancetta and reserve. Add the onion and cook for 5 mins. Stir in the garlic and red pepper flakes and cook until fragrant, about 30 secs. Add the tomatoes, 1/2 tsp salt, 1/2 tsp black pepper, and the cooked pancetta. Simmer, uncovered, over medium-low heat until the sauce thickens, about 15 mins. Stir in the cheese and season with salt and pepper, to taste.
For the meatballs:
1) Position an oven rack in the lower third of the oven. Preheat the oven to 200°C/ gas mark 6. Line a baking sheet with parchment paper. In a large bowl, combine the onion, 15g parsley, 65g Parmesan, breadcrumbs, egg, tomato ketchup, garlic, red pepper flakes, 1 tsp salt, and 1/2 tsp pepper. Add the beef and veal. Using your hands, combine all ingredients gently but thoroughly. Shape the meat mixture into 16 (4cm diameter) meatballs and place on the prepared baking sheet. Make a hole in the center of each meatball and place a cube of mozzarella inside. Reform the meatball so that the mozzarella is completely covered with the meat mixture. Bake the meatballs for 15 mins until cooked through.
2) Bring a large pot of salted water to a boil over high heat. Add the pasta and cook until tender but still firm to the bite, stirring occasionally, about 8 to 10 mins. Drain and place in a large serving bowl. Add the meatballs and sauce. Toss gently and season with salt and pepper, to taste. Sprinkle with the remaining parsley and Parmesan.