In a large bowl, combine the soy sauce, sesame oil, honey, gochujang, minced garlic, grated ginger, and black pepper. Stir well to create the marinade.
Add the thinly sliced beef sirloin to the marinade and toss to coat all pieces evenly.
Cover the bowl with plastic wrap and refrigerate for at least 1 hour, preferably 2 hours, to allow the flavors to develop.
Preheat a grill or grill pan over medium-high heat.
Remove the beef from the marinade, letting excess drip off, and place the slices on the grill in a single layer.
Grill the beef for 2-3 minutes on each side or until cooked through and slightly charred.
Transfer the grilled beef to a serving plate and sprinkle with toasted sesame seeds and sliced green onions.
Serve immediately with steamed vegetables or a side of rice for a complete high-protein meal.