Whisk 3 tbsp soy sauce, 1 tbsp gochujang, 1 tbsp brown sugar, 1 tbsp toasted sesame oil, 1 tbsp rice vinegar, 2 cloves garlic and 1 tbsp fresh ginger in a bowl. Toss 1 lb beef sirloin with 2 tablespoons of the sauce and let it marinate while you prepare the vegetables, about 10 minutes.
Freeze the steak for 20 minutes before slicing. It firms up, so you can cut paper-thin slices.
Julienne2 carrots.
Heat 1 tbsp vegetable oil in a large skillet or wok over high heat until it shimmers. Spread the beef out in one layer and let it sear without moving for 1 minute, then stir-fry until browned, about 3 minutes. Transfer to a plate.
Add 1/2 green cabbage, the carrots and the scallion whites to the pan. Stir-fry until just tender but still crisp, about 4 minutes.
Return the beef to the pan with the rest of the sauce and toss for 1 minute, until glossy.
Spoon over 4 cups cooked rice and top with the scallion greens and 1 tbsp sesame seeds.
Notes
Gochujang brands differ in heat. Start with 1 tablespoon and add more to taste.Ground beef works too: brown it instead of the sliced beef and break it up with a spoon.