Sprinkle chicken with lemon juice and set aside. Preheat oven to 400 degrees. In frying pan, melt half the butter over medium heat. Add chicken and cook for approximately 8 minutes on each side until lightly browned. Remove and set aside. Grease cooking dish with half of remaining butter.
Melt rest of butter over medium heat. Stir in flour and cook, stirring for 1 minute. Add chicken stock gradually, stirring constantly. After absorbed, bring to boil, stirring constantly. Stir in wine. Cook until thick. Beat 4 tablespoons of hot sauce into beaten egg yolk, one spoon at a time, slowly. Stir egg mixture into hot sauce.
Cook over very low heat (so it won't curdle) until thick. Add salt, pepper, cayenne, nutmeg, mushrooms and onions. Cover with foil or lid. Bake at 400 degrees for 30 to 40 minutes until done.
