1. In a large pot, bring 1.5 liters of salted water to a boil with the chicken broth tablet. When the water starts boiling, add the chicken pieces and let cook for fifteen minutes.
2. In a Dutch oven, melt 20g of butter, add the onions and bacon, and let simmer slowly.
3. Remove the chicken from the heat, add the pieces to the Dutch oven, and reserve the juice.
4. In a saucepan, melt the remaining butter, add the flour and stir for two minutes, then gradually add half of the chicken cooking juice to obtain a creamy sauce that is poured into the Dutch oven. Add the button mushrooms, salt, and pepper.
5. Let simmer over low heat for thirty minutes.
6. In a bowl, whisk the egg yolk with the heavy cream and lemon juice.
7. To serve, drain the vegetables and chicken with a slotted spoon and arrange them in a hot serving dish.
8. Pour the contents of the bowl into the Dutch oven, mix for one minute over low heat, then pour the sauce over the dish, sprinkle with chives, and serve immediately.