To prepare lamb on the grill, start by cleaning and oiling the grill. Preheat it for indirect heat, with the fire on one side. In a bowl, mix mustard, salt, pepper, rosemary, brown sugar, garlic, and apple cider vinegar.
Rub this mixture generously on the lamb racks and let them rest for 20 minutes. When ready to grill, place the lamb fat side down on the hot side for 3 to 6 minutes to get grill marks.
Then, transfer it to the cooler side and cook with the lid down for 15 to 25 minutes until the internal temperature reaches 130°F.
Remove from the grill, wrap in foil, and let it rest for 10 minutes before slicing along the ribs to serve. Serve with Cayenne Maple Roasted Veggies.