Peel onion, carrot, keep the best celery stalks for the garnish and put the rest 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 it 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 5 cm pieces, halve and cut into 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 so that no lumps form, you can use an immersion blender if necessary.
Add the vegetables and let cook for 2 minutes.
Whisk the yolk into the cream and add, mix well and place the thighs in.
Heat everything together.
Serve with potatoes and some finely chopped parsley.