Place lamb, meat side down, on a broiler pan coated with cooking spray.
Cut a 1-inch-deep slit between each rib on the underside of each rack; stuff 2 garlic slices into each slit. Turn lamb, meat side up; coat with cooking spray.
Sprinkle with pepper and rosemary, pressing gently to adhere. Cover and marinate in refrigerator 8 hours or overnight.
Preheat oven to 45
Sprinkle lamb evenly with salt.
Bake at 450 for 20 minutes or until a thermometer registers 145 (medium-rare) to 160 (medium). Cover with foil; let stand 10 minutes before slicing each rack into 8 pieces.