Step 1: Prepare the lamb: Using the tip of a sharp paring knife, make slits about an inch deep all over the surface of the lamb. Insert a sliver each of garlic and ginger into each slit. Place the lamb in a nonreactive roasting pan and set aside while you prepare the glaze.
Step 2: Make the glaze: Combine the Worcestershire sauce, soy sauce, brown sugar, Dijon and Chinese-style mustards, lemon juice, oil, and minced garlic and ginger in a small, heavy saucepan and bring to a boil over medium heat, stirring to dissolve the sugar. Cook the glaze until thick and syrupy, about 3 minutes, stirring frequently to prevent sticking. Remove the glaze from the heat and taste for seasoning, adding salt and pepper as necessary. Let cool to room temperature.
Step 3: Pour half of the cooled glaze over the lamb in the roasting pan, brushing to coat it on all sides. Cover and let marinate, in the refrigerator, for 3 to 8 hours (the longer the better). Refrigerate the remaining glaze, covered.
Step 4: Set up the grill for indirect grilling (preferably, you’ll have built a wood fire; let it burn down to glowing embers), place a large drip pan in the center, and preheat the grill to medium. Toss the wood chips on the coals.
Step 5: When ready to cook, place the lamb on the hot grate over the drip pan and cover the grill. Cook the lamb until done to taste, 1 to 1-1/4 hours for rare (internal temperature of 120 to 125 degrees); 1-1/2 to 1-3/4 hours for medium-rare (130 to 135 degrees); 2 hours for medium (145 to 150 degrees). Start brushing the lamb with the remaining glaze during the last 45 minutes of grilling, brushing it two or three times. If using a charcoal grill, you’ll need to add 10 to 12 fresh coals to each side every hour.
Step 6: Transfer the lamb to a cutting board and brush it one last time with glaze, then let rest for 10 minutes before carving. While the lamb rests, heat any remaining glaze to serve as a sauce with the lamb.
