Trim the fat on the lamb so there is only ~1/8" of fat. We want to keep some fat because we aren't using any oil.
Mix the sumac and garlic powder in a bowl. Liberally season the two racks of lamb on all sides with the seasoning.
Lay the two racks fat side up on a baking sheet. Bake 15 minutes, rotate the pan, and bake another 10 minutes.
While the lamb is in the oven, mix your lettuce, radishes, and cucumber in a bowl. Squeeze with the juice of the lime. Drizzle 1-2 tbsp of olive oil, and season with salt and pepper. Toss to coat.
Use a meat thermometer and take the lamb out of the oven at 135F. It should reach 145F before serving as it rests. Let the lamb rest for 5 minutes before cutting into it and serving.You do not need to tent the lamb, but you can if you would like. Then, cut the rack between every other rib bone to serve. Stack the pieces upright, using the bones to hold each other up for a showstopper presentation. See photo for inspiration.