Make the lamb: In a medium bowl, mix together the yogurt, garlic, ginger, garam masala, turmeric, cayenne, 2 teaspoons of salt, the pepper, and ground cashews.
Set the lamb racks in a rimmed baking dish and poke all over with a fork. This will allow the flavors of the marinade to season the meat all the way through.
Season the lamb with the remaining teaspoon of salt, then rub the marinade into the meat. Cover with plastic wrap and refrigerate for 6–12 hours.
Arrange a rack in the center of the oven. Preheat the oven to 450°F (220°C).
Transfer the marinated lamb, fat-side up to a roasting pan and weave the racks together so they are partially upright. Roast for 15 minutes, then turn the racks over and cook for another 10 minutes, or until the internal temperature reaches 130°F (55°C). Let rest for 10–15 minutes.
While the lamb rests, make the yogurt sauce: In a medium bowl, stir together the yogurt, shallots, sumac, cilantro, salt, and pepper. Garnish the yogurt sauce with the pomegranate seeds and mint leaves.
Carve the lamb between the rib bones into individual chops. Transfer to a serving platter and serve with the yogurt sauce alongside.