Prepare the marinade: In a large mixing bowl, whisk together the soy sauce, gochujang, mirin, ginger, garlic powder or paste, sesame oil, neutral oil, and brown sugar.
Marinate the steak: Add the thinly sliced beef to the marinade. Mix thoroughly to coat all the pieces. Cover and refrigerate for 2 hours.
Preheat the pan: Warm a serving bowl in the oven to keep the first batch of cooked beef warm. Heat a wok over very high heat until smoking hot.
Cook the steak: Work in two batches. Add half of the beef to the hot wok and spread it out in a single layer. Let it sear undisturbed for about 3 minutes to caramelize, then stir and cook for another 2–3 minutes until browned and slightly crispy. Transfer to the warm bowl and repeat with the remaining beef.
Garnish and serve: Sprinkle with sesame seeds and serve immediately with boiled rice. Add chopped cucumber and tomato on the side for freshness.
