Wash the chicken, pat dry and divide into 8 pieces. Clean or peel the leek and carrot, wash and chop roughly. Peel onion, lard with laurel and cloves. Wash thyme
Put the chicken, leek, carrot, onion, peppercorns, 1 teaspoon salt and half of thyme in a large pot with about 1 1/2 l water. Bring to the boil and simmer covered for about 40 minutes. Skim off the resulting foam several times.
Clean and wash the mushrooms and cut them into thin slices. Remove the chicken. Sieve the broth, measure out a good 1/4 l (use the rest for other purposes)
Heat 3 tablespoons of butter in a large frying pan. Fry the chicken parts in it until golden brown all around. Add the mushrooms and fry for 10-15 minutes
Chop the rest of the thyme. Heat 3 tablespoons of butter in a saucepan. Sauté the flour in it. Stir in 1/4 l stock, champagne and thyme. Bring to the boil and simmer for about 5 minutes. Stir egg yolk and cream until smooth. Remove sauce from the stove. Stir in egg yolk (do not boil any more!), season to taste. Arrange everything. Served with: baguette