Set large skillet/cast iron (something large that can hold the liquids to come) over high heat and preheat for 3 minutes. Add in butter and beef (seasoned with salt and pepper), cook for 3-4 minutes, then add in onions. Cook for another 3 minutes, then remove everything from the pan.
In same pan still over high heat, add in mushrooms and saute for 5 minutes. Add in soy sauce and remove from pan.
Deglaze pan with the beef stock, and then add in meat and onions back. Bring to a simmer. As it heats, combine the sour cream and mustard in a seperate bowl.
Once sauce has come to a simmer, stir in the cornstarch (loosened with a bit of water), followed by the sour cream/mustard. Let thicken for 2 minutes, then serve over cooked pasta, garnished with the mushrooms