Place the thinly sliced beef in a large mixing bowl.
1 pound thinly sliced beef sirloin or ribeye
Finely mince or puree the onion using a blender or food processor.
½ medium onion
Add the onion to the beef along with soy sauce, brown sugar, garlic, sesame oil, and black pepper. Mix thoroughly until the beef is evenly coated.
¼ cup soy sauce, 2 tablespoons brown sugar, 4 cloves garlic, 1 tablespoon sesame oil, ¼ teaspoon black pepper
Cover and refrigerate for at least 30 minutes. For deeper flavor, marinate for up to 4 hours.
Heat a large skillet or grill pan over medium-high heat. Add vegetable cooking oil.
1 tablespoon vegetable cooking oil
Cook the beef in batches, spreading it out in a single layer to ensure proper searing.
Cook for 2 to 3 minutes per side, until browned and slightly caramelized.
Transfer to a serving dish and garnish with sliced green onions and sesame seeds.
1 teaspoon sesame seeds, 2 green onions
Serve hot with steamed rice and lettuce leaves if desired.