Place steak, peppers, onions and marinade in a 1 gallon resealable food storage bag; seal. Toss to coat; refrigerate several hours or overnight.
Preheat grill to medium heat. Grill steak, turning once, 8-16 minutes (depending on thickness)or until desired doneness is reached. (145F for medium-rare; 160F for medium) Grill peppers and onions until desired doneness.
Remove from grill and allow steak to rest 5-10 minutes. Thinly slice steak; slice peppers and onions. Serve with fajita toppings.