Use your fist, rolling pin or mallet to flatten the 1.2 lbs scotch fillet steak/boneless ribeye to about 1/3" thick. Slice into 1/5" strips, cutting long ones in half and discarding excess fat. This does not have to be done if using sandwich steaks.
Sprinkle with a pinch of salt and pepper.
Heat 1 tbsp oil in a large skillet over high heat. Scatter half the beef in the skillet. QUICKLY spread it with tongs. Leave untouched for 30 seconds until browned. Turn beef quickly. Leave untouched for 30 seconds to brown. Immediately remove onto a plate. Don't worry about pink bits and that it will be raw inside.
Add remaining 1 tbsp oil and repeat with remaining beef.
Turn heat down to medium high. Add 3 tbsp butter and melt. Add 1 large onion, cook for 1 minute, then add 10 oz mushrooms.
Cook mushrooms until golden. Scrape bottom of fry pan to get all the golden bits off (this is flavor!).
Add 2 tbsp flour, cook, stirring, for 1 minute.
Add half the 2 cups beef broth while stirring. Once incorporated, add remaining broth.
Stir, then add 2/3 cup sour cream and 1 tbsp Dijon mustard. Stir until incorporated. Don't worry if it looks split; the sour cream will "melt" as it heats.
Bring to simmer, then reduce heat to medium low. Once it thickens to the consistency of pouring cream (3-5 minutes), adjust salt and pepper to taste.
Add beef back in, including plate juices. Simmer for 1 minute, then remove from stove immediately.
Serve over 10 oz pasta or egg noodles, sprinkled with chopped chives, if desired.
