Clean the onions and chop them into slices. Heat about 5 table spoons of olive oil in a wok, fry the onions until lightly brown, and then add about 0, 7 liter of water. Simmer until the onions are really soft. Add salt and pepper according to taste. If you like you can use ready made vegetable broth to season the soup.
Fill the soup into 4 fireproof bowls. Top each bowl with toasted bread and spread the grated mozzarella cheese over the bread. Brown the mozzarella cheese evenly under the grill and serve.