Preheat the oven to 200°C.
For the meatballs, combine the pork, salt, pickled peppers, eggs and bread in a large bowl.
Using an ice cream scoop, scoop 24 meatballs onto a baking sheet greased with the olive oil and bake for 17 minutes.
For the sauce, heat the oil in a pan and add the onion, pork, chilli flakes, and salt and cook for 15 minutes. Add the tomato paste and plum tomatoes with their liquid and cook for an additional 30 to 40 minutes, crushing the plum tomatoes as the sauce cooks.
For each serving, toast a brioche roll, cut side up, under the broiler until golden brown, and ladle sauce over each half of the roll.
Place a thick slice of mozzarella on the top half of the roll, and "smash" one meatball between a set of tongs and place it on the bottom half of the roll.
Repeat with a second meatball. Melt the cheese under the broiler and serve.