Peel ui, carrot, keep the most beautiful celery stems for the garnish and the rest goes in the broth.
Cut coarse and put all the ingredients for the broth, with the buttocks in a large cooking pot.
Put completely under water, season with salt and pepper.
Leave up, reduce fire and let it simmer for 25 minutes.
Allow to cool in broth, remove buttocks and sieve broth.
Peel the carrots, cut into fine strips, peel the fibers of the celery branches, cut into fine strips, also cut the leek into pieces of 5 cm and in half and again into strips.
Also cut the fennel into large pieces.
Melt butter in a large pot, let it foam and stir in the flour, let it fry.
Deglaze with broth and stir firmly so that you don't get lumps, possibly put the hand blender in it.
Add the vegetables and let it boil for 2 minutes.
Beat the yolk under the cream and add, mix well and put the buttocks in them.
Warm everything together.
Serve with potatoes and some finely chopped parsley.