In a bowl, combine the soy sauce, brown sugar, sesame oil, minced garlic, and grated ginger. Whisk together until the sugar is dissolved.
Add the thinly sliced beef to the marinade. Mix well to coat the meat evenly. Let it marinate for at least 10 minutes.
Heat the vegetable oil in a skillet over medium-high heat.
Once the oil is hot, add the marinated beef (discard excess marinade) and cook for about 5-7 minutes, stirring occasionally, until the beef is cooked through and browned.
Stir in the chopped green onions and cook for an additional minute.
Serve the Korean beef over cooked rice, garnished with sesame seeds if desired.
For extra flavor, consider adding sliced bell peppers or carrots into the skillet while cooking the beef.
Adjust the amount of brown sugar based on your preferred sweetness level.
Leftovers can be stored in the fridge for up to 3 days. Reheat before serving.