In a small saucepan, heat marmalade and mustard over low heat. Whisk in butter until melted. Set aside 1/4 cup glaze for serving.
Broil steaks 4 in. from the heat for 5-7 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°), basting occasionally with remaining glaze.
Spoon reserved glaze over steaks; sprinkle with onions.