Preheat the oven to 400°. In the bowl of a mini processor, finely chop the pistachios with the thyme and rosemary. Add half of the olive oil and process to a paste; season with salt and pepper. Scrape half of the pistachio paste into a small bowl and stir in the remaining olive oil.

Coat the lamb with half of the remaining pistachio paste. Wrap the pancetta slices around the lamb, between the bones, leaving the bones exposed. Spread the remaining pistachio paste over the pancetta and set the rack in a small roasting pan. Roast the rack for about 40 minutes, or until an instant-read thermometer inserted in the center of the meat registers 130° for medium-rare. Transfer the lamb to a cutting board and let rest for 5 minutes. Reserve the pan drippings.

Meanwhile, spoon 1 teaspoon of the rendered pancetta fat from the roasting pan into a medium skillet and heat until shimmering. Add the scallions and cook over high heat until softened and browned in spots, about 4 minutes. Carve the lamb rack into four 2-chop servings and transfer them to plates along with the scallions. Drizzle the pistachio pesto all around and serve right away.