PREHEAT oven to 450˚F or grill to medium-high. Cut steak lengthwise in half and then crosswise into ⅛ inch thick strips. Sprinkle steak strips with salt and pepper; set aside.
CENTRE ¼ of onions on each sheet of ALCAN® Non-Stick Baking Foil with non-stick (dull) side toward food. Top with steak strips and cheese.
BRING up foil sides. Double fold top and ends to seal packet, leaving room for heat circulation inside. Repeat to make four packets.
Cook or grill steak 8-18 minutes to desired doneness.