Melt butter in pan and saute onion until transparent. Saute beef in onion and butter until little red showing. Remove from pan and separate meat from sauce.

In pan used for beef, saute mushrooms until cooked. If any juice, remove to juice dish. Add beef to mushrooms and season with salt, pepper and nutmeg.

Separately mix sour cream with enough sauce to make rich, smooth sauce. Pour over meat and mushrooms, and heat through.

Great over spaetzle.