Remove the stems from the mint and place the leaves in a food processor with the pistachios, garlic, oil, zest and juice. Pulse the mixture until finely chopped and paste like. While doing so, occasionally scrap down the sides of the bowl with a spatula.
Preheat the oven to 425°F. Cut the lamb into 2, 4 rib racks and set in a roasting pan, bone-side down. Divide and spread the pistachio mixture on the meaty portion of the lamb. Season the lamb with salt and pepper.
Roast the lamb 22 to 25 minutes for rare, 25 to 28 minutes for medium rare, and 30 minutes for medium. Set the lamb on a plate, tent with foil and rest 5 minutes. (For rare, when checked with an instant-read meat thermometer, the centre of the meat should be 125°F. For medium-rare it should be 130°F, and for medium 140°F.)
Transfer the lamb to a plate, cover loosely with foil and rest for 5 minutes.
Place the reduction, stock, honey and cornstarch in a small bowl and whisk until smooth. Remove excess fat from roasting pan, and then set on the stovetop over medium-high. Add to the stock mixture to the pan and simmer until a thickened sauce forms, about 2 minutes. Season the sauce with salt, if needed.
Make a pool of sauce on each of 2 plates. Set a lamb rack on each plate and serve.