Flatten the steaks with a rolling pin to about 1/3" thick. Slice into 1/5" strips (cut long ones in half), discarding excess fat.
Sprinkle with a pinch of Member’s Mark™ Sea Salt and Member’s Mark™ Black Peppercorn Grinder.
Heat 1 tbsp oil in a large skillet over high heat. Place half the beef in the skillet, and spread it across the skillet evenly. Leave untouched for 30 seconds until browned. Turn beef quickly; leave untouched for 30 seconds. Remove onto a plate. (The beef will be cooked again later, so don’t worry about the pink bits/rawness.)
Add remaining 1 tbsp oil and repeat with remaining beef.
Turn heat down to medium high. Add butter; melt. Add onions; cook for 1 minute, then add mushrooms. Cook mushrooms until golden. Scrape bottom of skillet to get all the golden bits off.
Add flour; cook, stirring, for 1 minute.
Add one cup of water and 1 tsp Member’s Mark™ Beef Bouillon while stirring. Once incorporated, add additional 1 cup of water and 1 tsp Member’s Mark™ Beef Bouillon.
Stir, then add sour cream and mustard. Stir until incorporated.
Bring to simmer, then reduce heat to medium low. Aer 3 - 5 minutes, adjust Member’s Mark™ Sea Salt and Member’s Mark™ Black Peppercorn Grinder to taste.
Add beef back in (including plate juices). Simmer for 1 minute, then remove from stove immediately.
Serve over pasta or egg noodles. Sprinkle with chives, if desired.