In a medium bowl, whisk together soy sauce, sesame oil, brown sugar, mirin, minced garlic, grated ginger, and black pepper.
Add the thinly sliced ribeye steak to the marinade and toss to coat evenly. Cover and refrigerate for at least 30 minutes, or up to 4 hours.
Heat a large skillet or wok over medium-high heat. Add a tablespoon of oil if your pan isn't non-stick.
Add the marinated beef to the hot skillet in a single layer (you may need to cook in batches to avoid overcrowding). Cook for 2-3 minutes per side, until browned and cooked through.
Add the thinly sliced onion to the skillet during the last minute of cooking and stir-fry until slightly softened.
Serve the beef bulgogi hot, garnished with chopped green onions and toasted sesame seeds. It's delicious served with rice.
Notes
Adjust the sugar and soy sauce to your preference. For a spicier version, add some gochugaru (Korean chili flakes) to the marinade.