Combine the garlic, the mint, the parsley, the pistachios, the bread crumbs and the lemon zest in a food processor. Blend until the garlic is finely chopped. Add 4 tablespoons oil and blend until a coarse paste forms. Transfer half of herb paste to small bowl and reserve
Sprinkle each lamb rack generously with salt and pepper. Preheat the oven to 450°F. Heat the remaining 2 tablespoons of oil in a heavy large skillet over high heat. Place 1 lamb rack, meat side down, in the skillet. Sear until golden, about 2 minutes; return to baking sheet, meat side up. Repeat with remaining lamb racks. Cool off for a couple of minutes.
Spread the Dijon mustard evenly over the lamb, then spread the herb and pistachio paste over the lamb racks. Place on to a lined baking sheet, meat side up.
Roast the lamb until a meat thermometer inserted into the center of the lamb registers 130°F for medium-rare, about 20 minutes. Transfer the lamb to a platter. Let it stand 15 minutes. Mix any pan juices into the reserved herb paste. Cut the lamb between the bones into individual chops. Serve with the herb and pistachio sauce.