Put the onion and some sunflower oil in a frying pan over high heat. Using a wooden spoon, sauté the onion for no more than 1 minute.
Add the steak to the pan and use the wooden spoon to move it around as you sear the meat for 1–2 minutes. The exterior of the meat should turn a brown colour but not necessarily be cooked.
Turn the heat down to medium.
Add the stock powder