Slice the pork tenderloin into very thin slices and place the meat in a large ziplock bag. Set aside.
In a bowl whisk together the soy sauce, sesame oil, brown sugar, garlic, ginger, red pepper flakes and one chopped green onion (reserve the remaining chopped green onion for garnish on the bowls).
Pour the marinade into the bag with the pork and allow to marinate in the fridge for at least 30 minutes, or overnight.
When ready to cook the pork, add 1 tablespoon of the vegetable oil to a large wok or skillet over medium high heat.
Once hot, add half of the meat, placing it in a single layer in the pan. Allow it to brown for a few minutes, without touching it.
Flip each piece of meat to the other side and allow to brown for another 2 minutes or so, until cooked through, but not overcooked. Remove to a plate.
Add remaining oil to the pan. Once hot, add remaining pork and cook in the same manner. Remove to a plate.
Freezing instructions: Place raw meat in a freezer safe bag and cover it with marinade. You may need to divide the portion in half and store it in two separate bags. Remove as much air as possible and freeze it flat. Thaw overnight in the refrigerator or quick-thaw in a big bowl of cold water when you get home from work. Bulgogi tastes best freshly cooked so freezing it raw is better.