Combine lemon juice, garlic, mustard, and Mansmith’s spice. Place the lamb in a large zip-top, plastic bag and pour the lemon mixture over it. Marinate in the refrigerator for 6 to 8 hours. Grill in a covered grill by indirect heat for about 1 hour or until meat thermometer reaches 145°. Remove from heat and let stand for 20 minutes more, until thermometer reaches 150 degrees. Here’s a sauce to baste your leg of lamb with during the last 20 minutes of grilling.