Slice the onion into fine rings and fry gently in the butter in a flameproof earthenware dih. Stir with a wooden spoon to avoid the onion sticking. Continue stirring while gradually sprinkling in the flour. Slowly add the warm stock and blend in thoroughly. Cook for twenty minutes and taste for salt towards the end.

Arrange the slices of toasted bread (use unsalted, country bread) in an ovenproof dish and pour the soup over. Grate, or slice the Gruyère or Fontina very finely and arrange evenly over the top. Put in the oven at 180° until it meals and serve when golden brown and crisp.

According to the great expert, Artusi, this soup can be made with either stock or milk; you could try both to see which you prefer. If liked, you could also replace the flour with 300g of peeled and diced potatoes.