Heat the butter and oil in a flameproof casserole and cook the chicken pieces (a few at a time) on each side until browned. Remove from the casserole and put aside. Add the shallots, garlic and sliced mushrooms to the pot and cook over a low heat for 8 minutes, then sprinkle in the flour and mix well so that it absorbs the oil. Slowly add a little wine, stirring constantly until blended. Repeat until all the wine and stock have been added, making a sauce. Then add the tomato purée, bring to the boil and reduce the heat. Return the chicken pieces to the pan and cover. Simmer for 30 minutes. Add the tarragon, parsley and chervil, mix through and serve.