For wholemeal breads au gratin, cut the onion into thin slices. Place 2 bacon slices on each bread slice and spread the onion over the bread.
Grill on a tray in the oven with the grill function at about 200 °C for about 2 minutes. In the meantime, cut the tomatoes into slices, spread on the bread, cheese on top and finish grilling for another 2-3 minutes.