In a bowl, whisk together soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, rice vinegar, and gochujang to make the marinade.

Place the thinly sliced flank steak in a resealable plastic bag or shallow dish, and pour the marinade over the steak. Massage the marinade into the steak until well coated, then let it marinate in the fridge for at least 30 minutes, or overnight for maximum flavor.

Heat a skillet or grill pan over medium-high heat. Remove the steak from the marinade and discard any excess marinade.

Cook the steak for 3-4 minutes per side, or until cooked to your desired doneness. Transfer the cooked steak to a cutting board and let it rest for a few minutes before slicing thinly against the grain.

Warm the tortillas in a dry skillet or microwave.

Assemble the tacos by placing a few slices of the cooked steak on each tortilla, followed by shredded lettuce, diced tomatoes, sliced green onions, and chopped cilantro.

Garnish with a squeeze of lime juice and serve immediately!