Skin lamb and trim away as much fat as possible. Score the flesh deeply with a criss-cross pattern and fill the slits with tarragon and season with salt and pepper. Place meat in a baking dish and rub with oil and sprinkle with onion. Pour white wine on top and marinate in the refrigerator for 4 hours, basting occasionally.
Preheat oven to 325° F . Roast lamb with marinade, basting frequently for 1 hour, or until meat is cooked to (about 5° F) below desired doneness. Remove lamb to a platter and cover loosely with foil to hold warm and finish cooking.
Pour marinade and cooking juices into a saucepan and bring to a boil. Reduce liquid by (about half), remove from heat and stir in whipping cream. Return to heat and keep warm over low heat; do not boil.
Carve meat as desired and serve with warm sauce.
Servings: 8