Place lamb in large bowl. Mix together vinegar, oil, salt, pepper, mustard & honey in 1 quart bowl; pour over meat. Add onion slices. Cover & refrigerate meat overnight or 12 hours, turning occasionally. Let stand at room temperature 1 hour before grilling. Drain lamb, reserving marinade & onions. Grill lamb 1- 1 1/2 hours, brushing with marinade. Place onion slices in foil pouch; place on grill during last 10-15 minutes. Serve as side dish.