Prepare mustard and rosemary mixture: Mix Dijon mustard, olive oil, lemon juice and rosemary in a dish.
Remove lamb from the refrigerator. Trim off any excess fat. Sprinkle salt and pepper over the racks of lamb. Cover the lamb with mustard mixture and let lamb come to room temperature, about 20 to 30 minutes. Reserve a small amount of the mustard mixture for basting while on the grill.
Preheat the grill to medium heat. Clean and oil the grill.
Place the rack of lamb on grill meat side up, with ribs away from direct heat. Do not place the lamb directly over the heat, in this instance we are actually roasting the lamb on the grill.
Cook for 14-20 minutes turning and basting occasionally with the mustard mixture.
Remove the lamb from the grill when internal temperature is 135-140 for medium rare.
Cover loosely with foil for 5 minutes before serving.