I found this recipe in a Russian cookbook back in the early 60's. According to that book, true stroganoff has a mustard sauce. I have changed the sauce a bit over the years and I usually make 1 1/2 or 2 times the sauce because it is just so good. Unfortunately, we were halfway through dinner when I remembered that I didn't take a photo. I'll try to remember the next time.
Trim all fat from meat and cut into 1/2" thick slices. Season with salt & pepper.
Melt 1 1/2 tablespoons of butter in a large saucepan and stir in flour. Cook for a minute or two and then add beef broth. Bring to a boil, then stir in mustard. Cook, stirring constantly until the sauce is thick and smooth.
In a skillet, heat remaining butter until very hot. Brown meat slices quickly on both sides and remove from pan. Add sour cream to mustard sauce and bring to a boil. Add meat to sauce, cover and keep warm for 15-20 minutes. You must keep it hot without simmering. (I turn on my oven and sit the pot on the rear of the stove top for this.)
Before serving, heat over high heat for 2-3 minutes. Serve over rice.