In a medium bowl, combine soy sauce, sesame oil, gochujang, brown sugar (or honey), minced garlic, grated ginger, rice vinegar, and optional chili flakes. Whisk until smooth and the brown sugar is dissolved.

Cut the steak into bite-sized pieces and add to the marinade, stirring to coat evenly. Cover and refrigerate for at least 30 minutes.

Heat a large skillet over medium-high heat. Add a tablespoon of vegetable oil and heat until shimmering.

Add the marinated steak pieces in a single layer and sear for 2-3 minutes without stirring.

Flip the steak pieces and cook for another 2-3 minutes, checking for desired doneness.

Transfer to a serving dish and sprinkle with toasted sesame seeds and sliced green onions. Serve hot.