600 g / 1.2 lb scotch fillet steak / boneless rib eye (Note 1)
2 tbsp vegetable oil , divided
1 large onion (or 2 small onions), sliced
300 g / 10 oz mushrooms , sliced (not too thin)
40 g / 3 tbsp butter
2 tbsp flour (Note 2)
2 cups / 500 ml beef broth , preferably salt reduced
1 tbsp Dijon mustard
150 ml / 2/3 cup sour cream
Salt and pepper
250 – 300 g / 8 – 10 oz pasta or egg noodles of choice (Note 3)
Flatten the steaks with your fist (or a rolling pin or a mallet) to about 3/4cm / 1/3′′ thick. Remove superfluous fat and slice into 5mm / 1/5′′ strips (cut long ones in half).
Add a bit of salt and pepper to taste.
In a large skillet, heat 1 tablespoon of oil over high heat. Spread half of the steak in the skillet with tongs as quickly as possible. Leave for 30 seconds, or until browned. Turn the beef as soon as possible (to the best of your ability!). Allow for a 30-second browning period. Remove to a plate as soon as possible. Don’t be concerned about pink portions or the fact that it will be raw on the inside.
Repeat with the remaining 1 tbsp oil and the remaining beef.
Reduce the heat to medium high. Melt the butter in the pan. Then add the onions and sauté for 1 minute before adding the mushrooms.
Cook the mushrooms until they are golden brown. Scrape the bottom of the pan to remove all of the golden pieces (this is what adds flavor!).
Cook for 1 minute, stirring constantly.
Half of the broth should be added while stirring. Add the remaining broth when it has been fully absorbed.
Stir in the sour cream and mustard, then serve. Stir until completely combined (don’t worry if it appears divided; sour cream will “melt” as it warms).
Bring to a low simmer, then turn off the heat. Adjust salt and pepper to taste once it thickens to the consistency of pouring cream (3–5 minutes).
Return the beef to the pan (including plate juices). Simmer for 1 minute before removing from the heat. (Observation 4)
If preferred, top with chives and serve over pasta or egg noodles.