First prepare the meatballs. Sauté the onion and garlic with seasoning in a hot oiled frying pan for about 5 minutes until soft and lightly coloured, adding the chilli flakes after a minute or two. Place the mince in a large bowl and add seasoning. Put the breadcrumbs in a separate bowl and moisten with the milk. Add seasoning, then stir the breadcrumbs and onion mixture into the mince and combine well. With wet hands, shape the mince mixture into large balls about 4cm wide. Transfer to a lightly greased plate or tray and chill for 30 minutes until firm.
Put a little oil in a frying pan and cook the meatballs over a medium-low heat with a dash of oil for about 10 minutes until coloured on the outside and cooked all the way through. Set aside to rest.
Meanwhile, combine all the salsa ingredients and mix well. Leave to one side.
Heat a grill until medium hot. Slice the bread rolls in half and toast the insides for a couple of minutes until golden. Remove from the grill and sit 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 grill 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.