Combine the olive oil, garlic, rosemary, lemon juice, lemon zest and salt & pepper in a small bowl.
Pour the mixture over the racks of lamb and let marinate at least 30 minutes, or up to overnight.
When ready to grill, allow the lamb to come to room temperature (about 20-30 minutes) while you preheat your grill. Season the racks with salt and pepper.
Prepare grill for high heat (450°F).
Cover the bones of the lamb with foil (to prevent charring) and place on grill, fat side down. Close grill and let cook for 7 minutes, flip and cook for another 5 minutes, or until the internal temperature reaches 145°F for medium-rare (or see below).
Remove from grill and let lamb rest 10 minutes. Cut into individual chops.
Notes
Rack of Lamb Cooking Temperatures:
Rare: 120-130 degrees (very red inside)
Medium-Rare: 130-140 degrees (bright pink inside)
Medium: 140-145 degrees (light pink inside)
Medium-Well: 145-150 degrees (barely any pink left) Well-Done: 150-160 degrees (no pink)