1. Cut the flank steak in half and place in a large, resealable plastic bag.
2. Add the beer, kimchi liquid, oil, and garlic to the bag. Seal and refrigerate for at least 30 minutes and up to 12 hours.
3. Remove the steak from the marinade. Pat dry and bring to room temperature. Reserve marinade.
4. Heat a large skillet on medium high. Cook the steak, turning once, until browned, about 7 minutes per side. Add half the marinade to the pan, lower the heat, and let simmer until slightly thickened.
5. Remove the steak from the pan, let rest for 10 minutes, and slice thin.
6. Warm the tortillas and top with steak, kimchi, and scallions. Add other garnishes, if desired. Serve immediately.

Suggested garnishes: sliced radish, sesame seeds, shredded fresh cabbage or carrots, cilantro, minced fresh chilies, hot sauce, or chopped pineapple.