1. In a large saucepan, bring 1.5 liters of salted water to a boil with the chicken stock. When the water begins to boil, put in the chicken pieces and cook for fifteen minutes.
2. In a casserole melt 20 g butter, add onions and bacon, let simmer slowly.
3. Remove the chicken from the heat, add the pieces to the casserole and reserve the juice.
4. In a saucepan melt the rest of the butter, add the flour and stir for two minutes then gradually add half of the chicken's cooking juices to obtain a creamy sauce that is poured into the casserole, add the mushrooms of paris then salt and pepper.
5. Simmer on low heat for thirty minutes.
6. In a bowl whisk the water yolk with the crème fraîche and lemon juice.
7. To serve, drain vegetables and chicken with a slotted spoon, arrange in a warm serving dish.
8. Pour the contents of the bowl into the pan and mix for one minute over low heat, then cover the dish with this sauce, sprinkle with chives and serve immediately.