In a small bowl, combine the garlic, rosemary, thyme, crushed red pepper and 1/2 cup of the olive oil. Put the lamb racks on a rimmed baking sheet or in a baking dish and pour the marinade over the racks. Cover and refrigerate, turning occasionally, for at least 6 hours and up to 24 hours.
Let the lamb stand at room temperature for 1 hour.
Preheat the oven to 275 degrees F.
Place a large cast-iron pan over medium-high heat and add the remaining 1 tablespoon olive oil. Brush the excess marinade off the lamb (reserve the marinade) and season with salt and pepper. Brown the lamb for 3 to 4 minutes, then remove to a plate. Add the reserved marinade to the pan and cook for 2 minutes. Let cool slightly.
Interlock the lamb bones from one rack with the other and return the racks to the pan meat-side down with the bones upright. Transfer to the oven and cook, basting with the pan juices about every 5 minutes, until an instant-read thermometer reads 135 degrees F (or to preferred doneness), 30 to 40 minutes. Rest the lamb for 20 minutes before carving.