In a heavy skillet melt half of the butter. Add onion and cook slowly until transparent. Remove from pan and set aside.
Raise heat to medium-high, add beef and cook briefly, turning to brown on all sides. Remove from pan and set aside with the onions.
Melt remaining half of the butter in the skillet and stir in mushrooms. Cover and cook 3 minutes. Season with pepper and nutmeg.
Whisk in sour cream but do not allow to boil. Return beef and onions to pan and heat through.
Serve over egg noodles or rice.Servings: 6