Preheat the oven to 400°F. Pat the rack of lamb dry with paper towels and let it come to room temperature for about 30 minutes.
Using a sharp knife, gently score the fatty side of the rack of lamb into diamonds for even roasting. Make sure not to cut through the meat.
In a food processor, pulse the pistachios until finely chopped but not pasty. Set aside. In the same food processor, pulse the garlic, parsley, cilantro, salt, black pepper, sumac, and olive oil until well combined but not pureed. This forms a thick, flavorful herb paste.
Spread the herb mixture over the fatty side of the lamb. Press the chopped pistachios onto the paste evenly to create a thick, even crust.
Place the racks on a baking sheet.
Roast in the preheated oven for 25-30 minutes for medium-rare to medium (internal temperature of 135°F). Tent the lamb with aluminum foil and let it rest for 10 minutes before slicing to allow the juices to redistribute.
Slice between the bones and serve on a platter. Garnish with fresh herbs for added presentation.