Slice off any large bits of fat from the beef, then thinly slice the meat, cutting any wide slices in half. Pat dry with kitchen paper, then generously season all over with salt and pepper.
Heat 1 tablespoon of oil in a large frying pan over a high heat. Once it’s nice and hot, add half the beef slices and quickly spread them out so each has direct contact with the pan. Leave to fry for 1 minute, then flip them over and fry for another minute. Remove and repeat with the second batch, adding a dash more oil if you need to. Don’t worry if they’re still a touch pink at this point. Remove the beef from the pan and set aside.
Lower the heat slightly to medium-high and melt in the butter. Add the onion and fry for 2 minutes until it just about starts to soften. Add the mushrooms and fry for around 5 more minutes until lightly browned and most of the moisture in the pan has evaporated. The mushrooms will immediately soak up any liquid, then they’ll begin to release it. Add the garlic and fry for another 30 seconds.
Stir in the flour, then gradually add the beef stock, stirring as you go to prevent lumps from forming. Stir in the cream, sour cream, Worcestershire sauce and mustard. Bring to a simmer and allow to gently bubble away until the sauce begins to thicken, around 5 minutes.
Stir in the beef alongside the resting juices and simmer for 1 minute, then remove the pan from heat and check for seasoning. You only need to simmer long enough to re-thicken the sauce and warm the beef through.
Serve with your carb of choice with a sprinkling of fresh chives.