Combine the first 3 ingredients; sprinkle over chicken. Slowly brown the chicken in butter; remove from pan. In the same pan cook the onion until tender. Move the onions aside and place chicken in pan. Add the mushrooms and tarragon and spoon with onions over chicken. Add 1/4 cup of water; cover and simmer until the chicken is tender, about 30 - 40 minutes. Makes 4 servings