Skin the leg of lamb and trim away all the outside fat and as much of the fat lying between the muscle tissue that you can reach. Score the flesh deeply with a criss-cross pattern and stuff the slits with the tarragon. Rub the meat with the oil and cover with the onion. Place in a suitable dish for marinating and pour over the white wine. Add a little salt and pepper and leave to marinate for about 2 hours in a cool place, basting occasionally. Roast
the lamb with the marinade, basting it frequently, in a warm oven 325 deg F until done. About 10 minutes before the meat is cooked, pour off the marinade and meat juices into a saucepan. Reduce the gravy to half its original quantity by boiling vigorously. Carve the meat into thin slices and add the resulting juices to the marinade. Arrange the meat on a serving dish and keep warm. Remove the gravy from the heat, stir in cream and carefully re- heat the sauce until it forms a medium-thick consistency. Pour the sauce over the lamb and keep warm.