1. 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.
2. Preheat oven to 450°.
3. 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.