Slice the beef thinly against the grain.
Combine soy sauce, brown sugar, sesame oil, garlic, and ginger in a bowl to make the marinade.
Add the sliced beef to the marinade and let it marinate for at least 30 minutes.
Heat vegetable oil in a large skillet over medium-high heat.
Add the marinated beef to the skillet and stir-fry for about 5-7 minutes.
Sprinkle sesame seeds and half of the chopped green onions over the beef.
Stir well and cook for an additional minute.
Remove from heat and garnish with the remaining green onions.
Serve the Korean beef over cooked rice.