Add the onion and sprinkle with cheese. You can put as much cheese as your conscience allows)
Carefully transfer the loaves to a baking sheet. From the sauce, they become softer, so you need to work quickly, or collect the sandwiches immediately on the baking sheet.
Put the baking sheet in the oven, preheated to 180°C, and bake the sandwiches for 5-6 minutes. The sauce should warm up and the cheese should melt.