Start by chopping the onions. Bring a pot of water to boil and cook the onions for 2/3 minutes. Remove from the water and set aside
Season the steaks with salt and pepper. Warm a pan with butter on high heat and when it starts to turn brown add the steaks. Cook for 2 min on each side for medium rare and 3 min for medium/ well done. Remove the steaks and cover with foil or a plate.
In the same pan add some more butter and needed and of the onions mixing well. Add the vinegar until it caramelises the onions. Then add the wine and the beef stock bring to simmer and cook until it thickens. Add some cornflour to help with thickening.
Add the meat back on the pan and warm the meat basting with the sauce and quickly serve. I went for potatoes but you could serve with mash too
