Whisk together hoisin sauce, vinegar, soy sauce, garlic, minced scallion, honey, and salt. Trim as much fat from lamb as you can. Put it in a shallow dish big enough to hold the lamb flat. Spread marinade over both sides of it. Marinate lamb, covered & chilled, at least 4 hours or overnight.
Prepare grill. Grill lamb on an oiled rack set 5-6 in. over glowing coals 12-15 min. on each side, or until meat term. registers 140°F. for medium-rare meat. OR broil under preheated broiler about 4 in. from heat for app. the same time on each side as for grilling.
Transfer lamb to cutting board and let stand 20 min. before carving. Holding sharp knife at a 45 degree angle, cut lamb across grain into thin slices. Garnish with scallion brushes.