Mix together the lemon zest, rosemary, salt and sugar. Season the racks on all sides with this mixture, cover and put in the refrigerator overnight. This acts like a quick cure, which will add flavor to the lamb when it is cooked.
Preheat the oven to 300 degrees F.
Put a large saute pan over medium heat and add the oil. Put one of the seasoned racks in the pan fat-side down and sear until golden brown, 3 to 5 minutes. Flip the rack and sear the other side for a few minutes. Remove to a rimmed baking sheet fat-side up, and repeat the browning process with the other rack.
Put the baking sheet with both racks on it in the oven and roast until the meat registers 120 degrees F to 125 degrees F in the center, 20 to 25 minutes. Let rest for 5 minutes before cutting into chops and serving.