For this, I used a heavy-based, deep-sided 12-inch skillet.
First of all, remove the beef from the marinade and set it in a colander so that the marinade drains from the beef back into its bowl. You’ll be adding that marinade to the sauce a little later — so don’t discard it. Give the beef a few minutes to allow most of the marinade to drain off. Good.
You’re now aiming to give batches of the sliced, marinated beef some fast, searing heat to lock in their juices, and to add a little — and I mean a little — charring color. I did this in three batches — that was just fine in my big, wide skillet.
So, set your skillet on high heat and add 5 tablespoons sunflower oil — the sixth and final tablespoon of oil comes a little later. As soon as the oil starts barely smoking, drop the heat to medium-high, and add the first batch of beef in a single, even layer.
Fry the slices for 30 seconds on each side, then quickly remove them with a slotted spoon and set aside on a plate. Try to leave as much of the oil as you can in the skillet for the next lot of beef. Repeat this fast-fry process for the remaining two batches. Time now to quickly make the sauce.
With the skillet set on a medium heat, add the sixth tablespoon of sunflower oil, then swiftly stir in the cayennes, ground bird’s eye chilies, garlic, ginger, and cumin. You’ll find that the mix is fairly thick and pretty dry — that’s fine. Keep stirring over that medium-high heat for about 90 seconds until the mix starts to give off its bloom of spicy aromas.
Now add the water and the marinade, and stir everything thoroughly until the sauce starts to bubble. As soon as that happens, turn the heat to high and add the sliced scallions / spring onions, the beef, and any juices from its plate. Give the skillet a couple of good stirs to coat the beef and the scallions with the sauce.
Keep the heat on high until the sauce begins to bubble rapidly. Turn off the heat and add the sesame oil. Give the skillet one good final stir and serve at once over cooked rice.