Combine marinade ingredients in large gallon ziplocked bag. Add in the whole flank steak. Seal and refrigerate for 30 minutes. Rotate bag, and let refrigerate another 30 minutes.
Heat oil in a skillet. Add in onions and peppers. Add in cumin, salt and chili powder (and crushed red pepper flakes if desired) Remove when onions and peppers are cooked, and onions and translucent. Arrange on one side of a dinner platter.
Add flank steak to the pan and sear on both sides for 2-3 minutes. Remove from pan and slice thinly against the grain. Arrange on the other side of the dinner platter.
Serve with desired toppings: salsa, cheese and warm flour tortillas.