Slice the flank steak into very thin strips. Discard fat.
Sprinkle the beef with a little salt and pepper.
Heat 1 tbsp butter in a large cast iron skillet.
Fry half of the beef in the skillet for about 2-3 minutes, turning occasionally.
Remove the beef from the pan and set aside.
Repeat frying with the remaining beef with one more tbsp of butter.
Remove the beef from the pan and set aside.
Add the remaining tbsp of butter to the pan.
Add the onions and garlic and cook for about 2 minutes.
Add the mushrooms.
Cook until the onions are translucent. Scrap the bottom of the pan to loosen up the bits.
Sprinkle the K.A Keto flour over the onion and mushroom mix and stir well.
Add the beef stock and stir well.
Stir in the Worcestershire sauce, mustard and sour cream and whisk into the sauce.
Bring to a boil and immediately reduce to a simmer.
Add the beef back to the pan and simmer for about 10 minutes.
Serve hot and enjoy.