Cut slits ⅓ of the way into the steak 2 cm apart. Flip the steak over and repeat.
Mix the onions, soy sauce and salt and surround the steak in onions. Cover and marinate in the fridge overnight
The next day, brush off all of the onions from the meat and set the onions aside. Season the steak with black pepper.
Preheat a frying pan over medium-high heat until very hot. Add the vegetable oil, and then add the steak. Fry on one side until nicely browned, and then flip and fry the other side - approx 2 - 3mins depending on the thickness of your steak.
When the steak is cooked to your liking, transfer it to a plate and rest it.Back to the onions: Turn down the heat and add the onions to the same frying pan. Cook for approx. 6 mins until they start to caramelise.
Add the soy sauce and continue cooking until the liquid has evaporated and the soy sauce, then add the red wine and cook of the alcohol - a couple of minutes.
To finish the sauce, add the butter and stir it in to thicken the sauce.
Serving suggestion:Cook some rice and chop some spring onions into it.Slice the steak thinly.Fill some bowls with the rice and spring onions and lay thinly sliced steak on to. Top with a good spoonful or two of the onion sauce.