Wash, quarter and finely dice the tomatoes. Dice mozzarella finely as well.
Grate parmesan. Wash sage, shake dry and chop finely. Mix the prepared ingredients. Season with salt and pepper.
Open the buns, but do not cut them. Spread the prepared filling evenly into the rolls.
Grill the rolls in the sandwich maker or brown them in a pan (if necessary use a second pan to weigh them down).