Preheat the oven to 375 degrees F. Line a large baking sheet with parchment paper. Combine the first 8 ingredients in a bowl. Moisten your hands with cold water. Divide and shape the meat into 12, 3” wide patties and set on the baking sheet. Set a cube of mozzarella in the centre of each patty. Lift up one of the patties. Pull up the sides of the meat to cover up the cheese. Now shape the meat into a ball, ensuring the cheese is completely encased by the meat. Set the cheese-stuffed meatball on the baking sheet. Repeat these steps with the remaining patties and cheese. You will end up 12 cheese-stuffed meatballs.
Roast the meatballs 30 minutes, or until cooked through. While the meatballs roast, place the tomato sauce in a large, wide skillet. Mix in the stock and pesto. Bring to a simmer, and then turn heat to low. Cover and reserve sauce on low heat until meatballs are cooked.
When meatballs are cooked, divide among 4 or 6 plates, top with sauce and serve.