Mix all the ingredients for the marinade: soy sauce, sesame oil, garlic, scallions / spring onions, ginger and brown sugar.
Clean the tenderloin from all big lumps of fat and silver skin. They will be quickly pan fried so the fat won't melt and the silver skin goes really tough. Cut the meat in strips of 1-2cm to prevent overcooking.
Let the meat marinade for 1-3 hours in the fridge, covered with cling film that is touching the surface of the marinade.
Remove the beef from the marinade and pass the remaining liquid through a sieve. Put the sauce in a pot and add the cornflour, previously mixed with cold water. Add the mix and let boil. Reserve.
Cook the meat strips in a really hot pan with vegetable oil. Let it sit still to let it caramelize and then flip. Be careful not to overcook the meat as it really can ruin the dish. Only put a few strips of meat at a time on the pan to make sure that they go golden. If you put too many in, then the heat from the pan is extremely reduced and they will boil instead of frying.
Add all your meat back into the pan and mix it with the sauce we thickened before.