It is very important to freeze the meat first, this aids you in cutting the meat into thin slices. After freezing, slightly defrost in the microwave - make sure the meat is still frozen, but workable. Cut the meat into thin slices, as thin as possible. Put the slices into the large mixing bowl and sprinkle brown sugar over it and mix with hands. Let it sit for about 20 minutes.
Meanwhile, prepare the marinade. Place onion into food processor or blender until liquid and pour into small bowl. Mix together soy sauce, brown sugar, black pepper and sesame oil. After mixing, taste it. You want to have a slightly salty, sweet taste. If it's too salty, add a bit more sugar. Add minced garlic, green onion and put into sugar/meat mixture. Pour both sauces into the garlic/onion meat mixture and mix well, using hands. It is important to mix thoroughly so the marinade is evenly distributed throughout the meat.
Refrigerate for at least two hours or overnight, the longer the better, not to exceed 3 days.
Cook in frying pan until the meat is cooked thoroughly.