Freeze the flank steak until it is firm but not solidly frozen. Using a sharp, non-serrated knife slice ¼ inch slices, across the grain, Place the sliced meat into a small bowl or zippered bag. Pour 1 cup of Korean Barbecue Sauce over the meat. Let it marinate for an hour, smooshing it midway to make sure all slices are covered.
Prep vegetables for cooking. Peel and slice onion. Stem, core, and slice peppers. Carefully tear leaves from lettuce. Rinse carefully and place on paper towels to dry.
Brush a cast-iron grill pan with 1 tbsp peanut oil. Heat until it begins to smoke. Add onions to the pan and grill until they just get a lit of brown. Don't overcook. The crunch is part of the magic. Remove the oinions and add the peppers. Cook until they just get soft and remove from heat.
Remove the meat from the marinade. Discard any sauce remaining the bag/bowl. Do not rinse or pat the meat dry. You want the sweet sauce left on the meat so that it caramelizes on the grill pan.
Brush 1 tbsp peanut oil on the grill and heat until it smokes. Add the meat to the grill pan, one slice at a time, acoss the raised portions of the grill pan. That will give you crispy grill marks while elevating a bit of the meat off the direct heat. It won't dry out this way. Leave a little room between slices. When you have put as many slices on as you can, go back and fip the first ones over. Repeat, turning each slice over briefly to cook the other side then remove from heat.
While the pan is still hot, add a tablespoon of Korean Barbecue sauce and toss the vegetables together on the grill pan to reheat them get some sauce on them. Remove from heat.
Serve with rice either on the wrap off as a side. Add meat and vegetables to the lettuce wrap and drizzle on some extra sauce. Roll it up and enjoy HOT! Have planty of napkins ready.