1. Lay lamb leg open on a cutting board.
2. Trim off visible fat.
3. Season lamb generously with salt and pepper; allow to sit at room temperature for 30 minutes prior to grilling.
4. In a food processor, add pistachios, mint, parsley, oil, vinegar, lemon juice, lemon zest, salt, pepper and chili flakes; blend until somewhat chunky.
5. Preheat grill to medium-high. Grill the lamb leg to medium-rare (remove from heat when thermometer reads between 130°F and 135°F), or to desired doneness. Rest the meat, lightly covered, for at least 10 minutes before serving. Slice meat across the grain into thin slices.