Combine the mustard, olive oil, lemon juice, garlic, rosemary, salt, and pepper in a small mixing bowl. Transfer the mixture to a large food storage bag, add the racks of lamb, seal, toss to coat, and let sit in the refrigerator for at least 4 hours or up to 12 hours.
Remove the lamb from the refrigerator and let come to room temperature while the grill heats up. Prepare the grill for direct and indirect heat and preheat to medium.
Remove the lamb from the marinade, discard the marinade, and pat the lamb dry. If desired, loosely cover the lamb bones with foil to keep them from burning.
Brush the grill grates with oil and place the lamb racks over direct heat, bone-side down, and sear for 2 minutes or until the meat starts to brown. Flip and sear the other side for another 2 minutes. Move the lamb racks to the indirect side of the grill, close the lid, and cook for another 15 minutes or until a thermometer inserted into the centre registers between 55-60°c for medium-rare. Remove the lamb from the grill and let rest 5-10 minutes before removing the foil from the bones and cutting the racks into chops.
