Quickly sear the beef strips on both sides (steak overcooks really fast so a minute or two is all you need). Remove from the pan with a slotted spoon, transfer to a plate, and set aside.
Return the pan to the heat and add 2 tablespoons butter. Once it has melted, add the onions and sauté until translucent. Add the mushrooms, and stir to combine. Continue sautéing for an additional 5-7 minutes, or until the mushrooms are cooked and the onions are soft.
Add the garlic and cook another minute. Add the flour and mustard powder and mix to coat well. Cook for another minute or two to cook the flour.
Add the wine or vermouth, and scrape the brown bits off the bottom of the pan into the white wine to deglaze the pan. Let the mixture cook down for another 3 minutes.
Add beef stock, and stir to combine. Let the mixture simmer for 5 minutes, stirring occasionally.
Remove from the heat and stir in sour cream until combined, and then return the cooked steak to the pan. Taste for seasoning and add salt and pepper if needed.
Serve over egg noodles, rice or potatoes, garnished with parsley if desired.
For extra flavor, marinate the steak strips in two tablespoons of soy sauce for 15 minutes before cooking. If you do this, be sure to dry well before searing and skip the seasoning.
Don't be afraid of a hot pan. To quickly sear the steak you want a really hot pan. A quick sear will ensure the steak isn't overcooked.
Be sure to cook the flour for a minute or two after adding it to the pan to get rid of its starchy flavor.
Use full-fat sour cream. Low fat and fat-free sour cream will liquefy when heated and will thin out the sauce.
Add sour cream off the heat. Boiling sour cream can cause the sauce to curdle.