The most important element in this soup is caramelizing the onions properly, to give depth of flavour and a good colour. Make this Lyonnaise soup only when you have a good beef stock, the other essential ingredient. Stock cubes just don’t give the same full-bodied taste.
Preheat the oven to 220°/425°C/gas 7.
Fry the onions gently in the butter or goose fat for 20 minutes, stirring frequently, until well browned.
Sprinkle over the flour and stir in. Cook for a further 2 minutes.
Pour in the stock. Season and bring to the boil. Lower the heat and simmer gently for 30 minutes.
Toast the baguette slices. Grate half the Gruyère and slice the rest very thinly on a mandoline grater.
Rub an ovenproof tureen with butter, sprinkle in enough grated cheese to coat the bottom and sides, then pack the toasts into it, layering them with the sliced Gruyère and finishing with more cheese on the top. Ladle over enough soup to soak the toasts, then put the tureen into the oven for 5 minutes, or until all the stock has been absorbed.
Add the rest of the soup and the brandy, sprinkle the top with the remaining grated cheese and return to the oven for a final 10 minutes.
Beat the egg yolk with the red wine and, when you take the soup bubbling from the oven, stir this mixture into the top just before serving.
© 1998 Alastair Little and Richard Whittington estate. All rights reserved.