Leave the onions whole. Cook them in a large quantity of boiling, salted water for 10 minutes, then drain them, place under cold running water to cool, and chop coarsely. Transfer to a bowl.

Chop the pancetta coarsely and sauté very lightly in the olive oil in a flameproof casserole, preferably terra-cotta. Add the chopped onions and sauté until they are golden, then add the hot broth. Cover the casserole and simmer for 35 minutes.

Toast the bread slices on both sides. Place one in each of 4 heated terra-cotta soup bowls.

Taste the soup for salt and pepper, then pour into the bowls. Sprinkle with Parmigiano and freshly ground black pepper, then cover the bowls with lids and allow to rest for 5 minutes before serving.