In a small bowl, whisk soy sauce, honey, ginger and chillies; transfer to a large plastic zip-top bag. Add lamb. Close bag, rubbing marinade into lamb. Refrigerate for at least 1 hour, or up to 8 hours, or even overnight, if time allows.
Remove lamb from bag. Grill or Broil on baking pan over high heat, fat side down, for 25 to 35 minutes, total, turning halfway through cooking, actual time depending on weight of lamb leg, and desired doneness. Check inside temperature regularly (145°F for medium-rare; 160°F for medium).
Remove lamb from grill/oven. Loosely cover with foil and let rest, about 15 minutes.
While lamb is resting, toss scallions in oil, season with salt and pepper, and sauté, turning occasionally, until charred, about 3 minutes.
Thinly slice lamb at a diagonal slant, serving lamb slices with charred green onions, sautéed sugar snap peas, and steamed rice.