Combine soy and fish sauces, sesame oil, crushed garlic, onion, ginger, water and 2 tsp brown sugar in a medium bowl. Whisk until sugar dissolves.
Thinly slice flank steak against grain into small strips.
Place marinade and steak in a ziplock bag and marinate for 2 hours, or best, overnight.
Heat large skillet over medium-high heat. Once hot, pour contents of bag into pan. Cook until meat is browned, about 6-8 minutes.
If there is an abundance of marinade remaining, drain off, reserving a few tablespoons in the pan. Turn heat to low, stir in remaining 2 tsp of brown sugar, along with 2 TBSP gochujang paste, mixing thoroughly.
Once combined, turn off heat and salt and pepper to taste.
Warm up flour tortillas, add your steak, and top with avocado, cilantro, cabbage, kimchi and a little drizzle of gochujang! Enjoy!