Use your fist, rolling pin, or mallet to flatten the steaks to about 3/4 cm / 1/3" thick. Slice into 5 mm / 1/5" strips, discarding excess fat. Sprinkle with a pinch of salt and pepper.

Heat 1 tbsp of oil in a large skillet over high heat. Scatter half the beef in the skillet, spreading it quickly with tongs. Leave it untouched for 30 seconds until browned. Turn the beef quickly and leave it for another 30 seconds to brown. Remove onto a plate.

Repeat with the remaining beef using the other 1 tbsp of oil.

Turn the heat down to medium-high. Add the butter and let it melt. Then add onions and cook for 1 minute before adding the mushrooms.

Cook the mushrooms until golden. Scrape the bottom of the pan to get all the flavor bits off.

Add the flour and cook, stirring for 1 minute.

Slowly add half the broth while stirring. Once incorporated, add the remaining broth.

Stir well, then add sour cream and mustard. Stir until mixed (it may look split at first, but the sour cream will blend as it heats).

Bring the mixture to a simmer, then reduce the heat to medium-low. Cook until it thickens to the consistency of pouring cream, about 3 to 5 minutes. Adjust salt and pepper to taste.

Add the beef back into the sauce (including the juices from the plate). Simmer for 1 minute, then remove from heat.

Serve over pasta or egg noodles, garnished with chives if desired.