To prepare the classic Tuscan carabaccia, start by peeling the onions and cutting them into thin slices. Drizzle a large pot with oil and place the onions, let them dry over medium heat for about 10 minutes. Remember to mix every now and then.
Once the onion has softened, blend with the wine and then add the sugar, then the sifted flour. Finally, the previously heated vegetable broth to cover the onions. Then season with salt and pepper and cook over medium heat for about half an hour.
Meanwhile, toast the slices of bread on the grill or, if you prefer, under the grill of the oven at a temperature of 200 ° C (392 ° F). Then set them aside.
At this point, grate the two cheeses and mix them together.
Once the broth has shrunk, transfer your soup, which must not be too liquid, into some bowls or oven tureens and place the toasted slices of bread on top.
Finally sprinkle everything with grated cheeses and place your onion soup under the oven grill. Wait until the cheeses are perfectly melted and golden; at that moment your carabaccia will be ready to serve. Do not forget to sprinkle it first with ground pepper which will give the whole a spicy note.