If you have the time, pots and inclination this dark sauce base is invaluable. Like stock it freezes well. Mixed with roasting juices of duck or beef it makes a good vehicle for quite complex flavours. It is made in two stages, preferably on separate days. Initially you make a good stock from bones. The next day you convert this stock into a more full-bodied sauce base. Don’t put any salt in either stock or sauce base.