Let the meat sit out at room temperature for about 30 minutes prior to cooking.
Combine the garlic, mustard, olive oil, rosemary, lemon juice and lemon zest in a small bowl.
Sprinkle the lamb evenly with salt and pepper. Coat the meat with most of the mustard sauce and reserve some for basting during grilling.
Preheat the grill to a medium setting.
Place racks of lamb on the grill but not over direct heat. Allow to cook for 7-10 minutes per side and turn once during cooking. Baste with reserved mustard sauce. [N0te #1: if you want to develop a nice crust and grill marks, resist the urge to keep flipping the meat over constantly during cooking. I know. Just flip once and step away. You can do it :)]
Remove the meat and allow to rest for 10 minutes prior to slicing. [Note #2: If you want tasty lamb, you’ve got to let it sit for 5-10 minutes before you hack into it or else the juices end up on your plate underneath a pile of dry meat. Go gnaw on a handful of macadamia nuts if you are too hungry to wait.]
For medium-rare, I grilled the lamb about 7 minutes per side. Your grill may vary.