Cut chicken into serving pieces, removing as much of the skin as possible. Season pieces with salt and freshly ground black pepper, and sauté in butter and olive oil until golden.
Sprinkle with finely chopped shallots and flour, and continue to cook, shaking pan, until shallots are transparent. Then add dry sherry, tomato concentrate, bouquet garni, sliced mushrooms and enough well-flavoured stock to cover. Cover the saucepan and simmer gently in a slow 325°-M2) until the chicken is tender.
To serve: arrange chicken pieces on a heated serving dish; skim fat from sauce; add lemon juice and cayenne pepper, to taste, and strain sauce over chicken pieces. Garnish with sliced mushrooms and croûtons or pastry crescents, and serve immediately.