Melt the
butter in a large pan, add the onion and fry for
5 minutes, without browning. Stir in the fennel,
then add the bouquet garni, stock and salt and
pepper to taste.
Bring to the
boil, cover and simmer for 30 minutes, until the
vegetables are very tender.
Remove the
bouquet garni and cool slightly. Sieve or work
in a electric blender until smooth, then reheat.
Mix the egg
yolk and lemon juice together with a few
tablespoons of the soup.
Stir the egg
mixture into the soup and serve immediately,
garnished with fennel leaves and croutons.