Sautè beef and onion in butter and oil (mixture of
both) in a heavy skillet (about 5 minutes). Slowly
add broth to beef, stirring well. Bring to a boil.
Combine sour cream, cream, paprika, pepper and
salt. Slowly stir sour cream mixture into beef
mixture. Turn heat to low and bring to a bare
simmer. Cook 15-20 minutes, stirring frequently and
never allowing mixture to boil. Beat one egg yolk,
then add a bit of the hot mixture to the yolks. Beat
again, add a bit more, then when yolk mixture is
heated, add to balance of sauce to thicken. In last
2-3 minutes add mushrooms if you are using them.

Serve over cooked low-carb pasta.