In a bowl, combine soy sauce, brown sugar, sesame oil, garlic, and ginger to create the marinade.

Add the sliced beef to the marinade, mixing well to ensure all pieces are coated. Allow marinating for at least 30 minutes.

Heat a skillet over high heat, then add the marinated beef and sliced onion.

Cook for about 5-7 minutes until the beef is browned and cooked through.

Stir in the chopped green onions and remove from heat.

Serve hot, garnished with sesame seeds.