Prepare the racks of lamb at least 1 hour (or up to 4 hours) before cooking: Chop together the garlic, salt, rosemary, and pepper to make a coarse rub. Rub the racks with the mixture, cover, and set aside. (If you are not going to cook them within 2 hours, refrigerate, then bring to room temperature an hour before cooking.)
Preheat oven to 425° F. Heat a roasting pan in the oven for 10 minutes. Pat the racks of lamb dry with paper towels and coat lightly with the olive oil. Place the racks flesh-side down in the pan. Roast 15 to 20 minutes or until an instant-read thermometer inserted in the thickest part reads 145° F for medium rare (the temperature will rise about 10° when the lamb is out of the oven). Transfer the racks to a carving board. Let rest for 10 minutes. Slice between the ribs to separate the chops for serving. Garnish with the fresh herb sprigs.