Toss rib pieces with the honey–soy–vinegar–mustard marinade; refrigerate 1–24 hr.
Preheat to 325°F; bake ribs covered 1¼ hr over 2 cups water, then uncovered 1 hr.
Boil reserved marinade 5 min until syrupy.
Brush ribs with ⅓ cup glaze; bake 15–30 min until browned.
Rest 10 min; slice, toss with remaining glaze, garnish scallion greens and salt, and serve.