Wrap steak in plastic wrap and freeze for 10-15 minutes for easier slicing. Remove plastic wrap and slice thinly with a very sharp knife. Place in a food storage bag, add 6 tablespoons Korean barbecue sauce, and marinate for at least 2 hours or overnight.
Combine mayonnaise with 1 tablespoon Korean barbecue sauce and refrigerate until serving time.
Heat 1 tablespoon oil in a cast iron skillet over medium-high heat. Working in batches, add the meat in an even layer, being careful not to over-crowd the pan, and cook for 1 minute per side.
When all the meat is cooked, add the remaining tablespoon of oil and cook the kimchi for about 2 minutes or just until it begins to char.
Spread the mayonnaise on both cut sides of each bagel. Place 2 lettuce leaves on the bottom half, top with the beef, then kimchi, and then microgreens or cilantro and top with the top portion of the bagel.