Heat the fat in a casserole, add the onions and cook for 15 minutes, until golden, stirring from time to time. Add the garlic, bouquet garni and stock, season, bring to the boil and simmer gently for 20 minutes.
Separate the eggs and mix the vinegar with the yolks. Put the bread into a tureen, pour the soup, then gently stir in the egg yolks and whites. Serve at once.