Place steaks in a large resealable heavy-duty plastic bag: pour teriyaki sauce over steaks. Seal bag and turn to coat. Refrigerate 30 minutes or up to 3 hours.
In a skillet, melt butter over medium heat. Cook mushrooms until golden brown. Add pepper strips and cook 3 minuets or until tender. Keep warm.
Coat grill rack with cooking spray before starting the grill. Drain and discard teriyaki sauce from steaks. Place beef over direct heat. Grill, covered, over high heat 15 minutes for medium doneness, turning once. Place steaks on serving plates. Stir onions into mushroom-pepper mixture; spoon over steaks.
Top loin steak may be labeled as strip steak, Kansas City steak, New York strip steak, ambassador steak or boneless club steak in your region.