Cut the filet tails into strips.
Add oil to a pre-heated saute pan and heat until you see the first wisps of smoke, or place butter in the pan and heat to bubbling. Add the meat and brown quickly on each side. Remove meat and keep warm.
Add the onions to the pan. When these 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 the sour cream and mix mustard with the other half. Add this to the mixture and stir until it is quite hot, but do not bring to a boil. Garnish with paprika.