Heat the oil or butter in a large skillet over medium-high heat.
Add the sliced beef and sear until browned, about 2–3 minutes. Remove from skillet and set aside.
In the same skillet, sauté onions until translucent, then add mushrooms and cook until softened.
Sprinkle flour over the mixture and stir well to coat.
Gradually stir in the beef stock and simmer for 2–3 minutes until slightly thickened.
Lower the heat and stir in sour cream and mustard. Season with salt and pepper.
Return the beef to the skillet and cook for another 2–3 minutes, just until heated through. Do not boil.
Serve over egg noodles or mashed potatoes, garnished with parsley if desired.