In a large bowl, combine soy sauce, sesame oil, honey, rice vinegar, minced garlic, grated ginger, gochujang, and black pepper. Whisk until well blended to create the marinade.
Add the thinly sliced sirloin beef 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 beef to absorb the flavors.
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 cooked beef to a serving platter and sprinkle with chopped green onions and toasted sesame seeds.
Serve immediately with steamed rice and your choice of Korean side dishes.