Trim beef tenderloin and cut into strips.

Heat butter or margarine in a large skillet over medium-high heat.

Brown the beef strips quickly in the hot butter.

Push the beef to one side of the skillet.

Add chopped onions and sliced mushrooms to the skillet.

Cook the onions and mushrooms until they are tender, but not brown.

Add beef broth to the skillet.

Heat the mixture to boiling.

In a separate bowl, blend yogurt with flour until smooth.

Stir the yogurt mixture into the boiling broth.

Cook until the sauce thickens.

Season with salt and pepper to taste.

Serve hot over rice.