Wash, quarter and finely dice the tomatoes. Dice mozzarella as well. Grate parmesan. Wash basil, shake dry and chop finely. Mix everything. Season with salt and pepper.
Open the buns horizontally, but do not cut through. Spread the filling into the rolls. Bake the rolls in portions in the sandwich maker or put them in a hot grill pan and weigh them down with a pan or pot. Roast for about 4 minutes on each side.