Peel onion, carrot, keep the nicest celery stalks for garnish and the rest goes in the broth.
Chop roughly and put all the ingredients for the broth, with the thighs, in a large cooking pot.
Cover completely with water, season with pepper and salt.
Bring to a boil, lower the heat and let simmer for 25 minutes.
Let cool in the broth, remove the thighs and strain the broth.
Peel the carrots, cut into fine strips, peel the fibers from the celery stalks, cut into fine strips, also cut the leek into pieces of 5 cm and then in strips.
Cut the fennel into large pieces as well.
Melt butter in a large pot, let it foam and stir in the flour, let it cook for a moment.
Deglaze with broth and stir vigorously to avoid lumps, you can use an immersion blender if necessary.
Add the vegetables and let boil for 2 minutes.
Whisk the yolk into the cream and add, mix well and place the thighs in.
Warm everything together.
Serve with potatoes and some finely chopped parsley.