How to Make It
Place leg of lamb on a large cutting board. Butterfly and open any parts of the lamb thicker than 1 inch. Sprinkle evenly with kosher salt.
Process garlic, scallions, yellow onion, celery, and ginger in a food processor until very finely chopped, about 10 seconds. Add soy sauce and sesame oil; pulse to combine.
Place lamb in a large baking dish; top with soy sauce mixture, and rub evenly over lamb. Cover, and refrigerate 12 to 16 hours, turning occasionally.
Preheat a gas grill to high (450°F to 500°F) on 1 side. Remove lamb from marinade, wiping off excess and discarding marinade. Place lamb on grill racks coated with cooking spray over lit side of grill. Cook until grill marks are well-formed, 8 minutes. Flip and grill until a thermometer inserted into thickest portion registers 120°F, another 8 minutes. If lamb starts to get too dark, transfer meat to unlit side of grill to finish cooking. Remove from grill; let rest 15 minutes.
Slice lamb against the grain, and arrange on a platter. Sprinkle with cilantro, peanuts, and salt; serve with lime wedges.