Be sure to remove the fell or filament and all excess fat from the lamb. Blend the mustard, soy sauce, garlic, herbs, and ginger in a bowl. Beat in the olive oil drop by drop to make a mayonnaise-like coating. Using a rubber spatula or brush, paint the lamb with the mixture and set it on the rack of a roasting pan. (For best flavor, coat the meat several hours before roasting.) Roast at 300 degrees F for 10 to 12 minutes per pound for medium rare (bone in), 13 to 15 minutes per pound for well done. Do not exceed a meat-thermometer reading of 155 to 160 degrees F, or the meat will lose much of its juice and flavor. For a boned leg, the cooking time more than doubles.
The coating for this roast leg of lamb is based on one of Julia Child's in her Mastering the Art of French Cooking. It both seals in the juices and adds a delightful flavor. If at all possible, use the fresh rosemary.