01 -
Using a mallet or rolling pin, flatten the steak to about 0.75cm thickness. Slice it into thin strips about 0.5cm wide, then season with some salt and pepper. Trim off any fatty pieces.
02 -
On high heat, warm up 1 tbsp of oil in a big skillet. Lay half the beef strips in a single layer using tongs, searing for about 30 seconds per side until browned with a slight pink inside. Remove these from the pan and repeat for the other half using the remaining oil.
03 -
Turn the burner down to medium-high. Drop in the butter and wait for it to melt. Toss in the onion slices and stir them for a minute. Follow up with the mushrooms, cooking until they're beautifully golden and making sure to scrape up any browned bits stuck to the pan for that extra flavor boost.
04 -
Dust the flour over your veggies and mix everything for about a minute. Slowly pour in just half of the stock, stirring steadily to get a lump-free mixture. Add the rest of the stock, the mustard, and the sour cream. Mix until everything's combined, let it come to a simmer, and then lower the flame to medium-low. Cook for 3-5 minutes until the sauce is thicker and creamy enough to pour. Taste and adjust with salt and pepper.
05 -
Add the beef (plus any juices from the plate) back into the pan. Let it simmer for just one more minute. Turn off the heat and serve on top of noodles or pasta, with a sprinkle of chives on top if you want.