From The Lobel Brothers’ Meat, an old cookbook owned by my mom.
Cut tenderloin into thin strips.
Place butter in large skillet and, when bubbling, add the meat and brown quickly on each side.
Remove meat from skillet and keep warm. Add the onion to the skillet.
When the onion are golden, add the mushroom slices and stir until they are wilted.
Return the meat to the pan, season with salt and pepper and reduce heat.
Add half of the sour cream. Mix the mustard with the other half of the sour cream. Add this to the mixture in the pan and stir until it is quite hot, but do not bring this to a boil.
Garnish with paprika. Serve over rice or egg noodles.