Start the tomato sauce: heat 1 tablespoon of olive oil in a medium saucepan over medium-high heat. Add the onion, garlic, and chilli and fry, stirring occasionally, until translucent — about 5 minutes. Add the canned tomatoes with their juices and cook for 10 minutes, stirring occasionally and breaking up the tomatoes with a spoon.

Using an immersion blender, blend it all together until smooth. Taste and add salt as needed. When it's properly seasoned, add the butter, Parmesan, and basil and stir until the butter melts.

Start the caramelized onions: heat 2 tablespoons of olive oil in a skillet over medium-high heat. Add the sliced onions and salt and stir to coat. After 5 minutes, reduce the heat to medium-low. Continue cooking, stirring every few minutes and adjusting heat as needed to prevent burning, for 25-30 minutes until the onions are deep golden brown and jammy.

In a large mixing bowl, tear the bread into small pieces and add the water. Let it soak for 2 minutes until the bread is soft, then mash it into a paste with a fork. Add the sausage meat, egg, Worcestershire sauce, onion powder, Parmesan, minced green chilli, and garlic. Mix with your hands until just evenly combined — do not overwork or the meatballs will be dense. Shape into approximately 24 meatballs, about 2 inches (5cm) in diameter.

Heat 2 tablespoons of olive oil in a large deep skillet or sauté pan (12-inch / 30cm) over medium-high heat. Brown the meatballs in batches of 4, turning to develop a golden crust on all sides — about 3-4 minutes per batch. Transfer to a plate lined with kitchen paper. Once all meatballs are browned, carefully drain the excess oil from the pan, leaving any browned bits on the bottom. Pour in the tomato sauce, scraping up any fond. Return the meatballs to the pan, cover, and simmer on the lowest heat for 25-30 minutes until the meatballs are cooked through (internal temperature of 160°F / 71°C).

To assemble: set your oven broiler / grill to high (260°C / 500°F or your oven's maximum broil setting). Position the rack 6 inches (15cm) from the element. Halve the sub rolls and place cut-side up on a rimmed baking sheet. Toast under the broiler for 1-2 minutes until lightly golden — watch closely as they can burn quickly. Place 3 meatballs on each bottom roll and spoon sauce generously over the top. Add a portion of caramelized onions and top with shredded mozzarella. Return to the broiler for 1-2 minutes until the cheese is melted and bubbling. Serve immediately.