Heat the broiler to high and toast the ciabatta rolls you cut in half, horizontally. (NOTE 1)
Roast the salami slices in a grill pan until they are crispy. No oil or butter needed.
Transfer the salami to paper towels to absorb the fat.
Place the onion slices in the grill pan and cook until brown and soft. (NOTE 2)
Cut the Mozzarella and tomato in thin slices and set aside.
Now build the sandwich:
Divide the salami over the 2 bottom halves of the ciabatta, then cover with the Mozzarella.
Place on a baking dish and slide the baking dish under a broiler until the Mozzarella melts. This will take about 5 minutes. (NOTE 3)
Take the sandwich halves out of the oven, add the onion, tomato and basil leaves.
Sprinkle with salt and pepper and serve.
Notes
Any other type of roll will do as well. Toast to add crunch and to avoid that the sandwich becomes soggy from the grease and melted cheese.
No need to add oil or butter when baking the onions. There is a little fat left in the pan from the salami to cook them in.
When melting the Mozzarella, it is better to go slow and take a little more time, then to go fast and burn the Mozzarella before it is thoroughly melted.