Step 1: Make the tandoori marinade: Place the garlic, ginger, turmeric, salt, mustard, pepper, cumin, coriander mace, cardamom, turmeric, and cayenne in a food processor and finely chop. With the motor running, add the oil in a thin stream. Process to a coarse paste. Work in the lemon juice. Stop the motor and add the yogurt. Run the processor in short bursts just to mix. Do not over-process or the mixture may separate.
Step 2: Prepare the leg of lamb: Using the tip of a paring knife, make a series of small holes in the lamb, 1/2 inch deep and wide and 1-1/2 inches apart. Place the lamb in a baking dish or large resealable plastic bag. Slather with tandoori marinade, forcing it into the holes with a spatula. Marinate in the refrigerator for 24 hours, turning several times so the lamb marinates evenly.
Step 3: Set up your grill for indirect grilling and preheat to medium-high (400 degrees F). Place a large drip pan in the center of the grill.
Step 4: When ready to cook, brush and oil the grill grate. Place the leg of lamb, fat side up, in the center of the grate over the drip pan and away from the heat. Lower the lid and cook the lamb until crusty and browned on the outside and cooked to taste, 1-1/2 hours for medium. (Indians like their lamb well done.) Halfway through cooking, start basting the lamb with melted butter, and continue basting every 15 minutes. Transfer to a platter and baste one more time before serving.
Step 5: Serve the lamb on a bed of sliced onion and cilantro sprigs, with lemon wedges for squeezing. Serve the Hellish Relish, Raita, and naan on the side.
