Season the flank steak with salt and pepper on both sides, and slice the steak very thinly (against the grain)
Stir flour, tomato paste, and beef stock until well combined
Melt 2 oz of butter in a heavy skillet (preferably cast iron) on med high
Saute onion and garlic for about 3 minutes
Add a pinch of salt and pepper, along with the sliced mushrooms, and saute for 5 minutes
Remove the vegetables to a plate and turn up the heat to high
Once the pan is really hot, melt the rest of the butter (2 oz) and add your steak (make sure pieces are not on top of each other) and sear each side for approximately 1 min. Remove to a separate plate
Turn the heat back down to med, and add the combined flour, tomato paste, and beef stock. Stir and scrape up all the bottom bits, add the onions, garlic, and mushrooms back, and then simmer for 3 min.
Add the steak back to the skillet, stir, then add in your sour cream and add salt and pepper to taste
Garnish with green onions
Notes
This is wonderful served over egg noodles, and feel free to add additional sour cream if you like.