In order to properly marinate any type of meat, you must use something to poke holes all over your meat. You can use a knife or a fork or anything that will penetrate the meat, so the marinade can soak into the inside of the meat. Puncture your entire flank steak with whatever tool you have on hand.
In a small bowl, whisk together the first 11 ingredients.
Place the flank steak in a resealable plastic bag. Pour in the marinade mixture.
Massage the marinade into the meat while in the bag. Seal the bag and refrigerate overnight.
Wrap the tortillas in aluminum foil and place them in the oven to warm.
Grill the flank steak on both sides until the internal temperature reaches about 135-140 degrees.
Remove from grill and let rest for five minutes.
Slice the flank steak against the grain and keep warm.
Grill the sliced onions and peppers until the desired doneness.
Return the flank steak to the grill and toss it with the vegetables.
Grill for about 3 minutes to warm.
Serve flank steak and vegetables wrapped in tortillas. Top with desired toppings.