In a large bowl, combine soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, and black pepper to create the marinade.
Add the thinly sliced beef to the marinade and mix well to coat all pieces evenly. Let it marinate for at least 15 minutes.
Heat a large skillet or grill pan over medium-high heat. Add marinated beef and cook for 3-4 minutes per side until fully cooked and slightly caramelized.
Remove from heat, garnish with chopped green onions, and serve hot with steamed rice or lettuce wraps.
Notes
For best flavor, marinate the beef overnight in the refrigerator. Adjust sugar amount according to taste preference.