To whiten chicken: clean and wash chicken; cut into serving pieces and put in a saucepan with juice of f lemon and enough cold salted water to cover. Bring to a boil; remove from heat and drain. Plunge into cold water for 5 minutes. Drain.
Place chicken in a saucepan with chicken stock to cover; add salt, freshly ground black pepper, bouquet garni, dry white wine and onions. Bring to a boil; skim; reduce heat and simmer gently until chicken is tender. Remove chicken pieces and onions to a clean casserole and keep warm in oven. Strain and reserve stock.
To make sauce: melt butter in the top of a double saucepan; add flour and stir until smooth. Pour in strained chicken stock and stir over water until boiling. Add sliced mushrooms and simmer gently for 15 minutes. Combine yolks with juice of ½ lemon (and a little double cream if desired), and stir in a little of the hot sauce. Pour egg and lemon mixture into double saucepan and stir until thick and smooth. Do not allow to boil or sauce will curdle.
To serve: place hot cooked chicken pieces and onions in a heated serving bowl; pour sauce over them and garnish with finely chopped parsley and croûtons of fried bread. Serve immediately.