First prepare the meatballs. Sauté the onion and garlic with salt and pepper in a hot oiled frying pan for about 5 minutes until soft and lightly colored, adding the chile flakes after a minute or two. Place the beef in a large bowl and season with salt and pepper. Put the breadcrumbs in a separate bowl and moisten with the milk. Season with salt and pepper, then stir the breadcrumbs and onion mixture into the beef and combine well. With wet hands, shape the beef mixture into large balls about 1½ inches wide. Transfer to a lightly greased plate or baking sheet and chill for 30 minutes until firm.
Put a little oil in a frying pan and cook the meatballs over medium-low heat with a dash of oil for about 10 minutes until colored on the outside and cooked all the way through. Set aside to rest.
Meanwhile, combine all the salsa ingredients and mix well. Set aside.
Heat a broiler to medium. Slice the bread rolls in half and toast the insides for a couple of minutes until golden. Remove from the broiler and set the meatballs on half the sliced rolls, pressing them down into the bread. Spoon over any cooking juices from the meatball pan. Top the meatballs with mozzarella and place under the broiler to melt the cheese. Once melted, spoon the salsa on top and sandwich together with the remaining halves of the toasted rolls.
Serve while still warm with any extra salsa on the side.