Melt butter in large
skillet, moderate heat. Add onion and mushrooms. Season with
salt and pepper, cook about 7 minutes stirring and tossing
until vegetables are tender. Empty into a bowl.
Stir yogurt and sour cream together and set aside. Cut chicken
in about 1 inch squares. Heat oil in skillet.
Season with salt and pepper and cook about 3 minutes, tossing
and stirring. Return mushrooms and onions with any juice
and cook 2 minutes more. Remove from heat and stir in yogurt
Do not cook further. Serve over noodles. Serves 4.