Cut 2 pounds tender beef into narrow strips 1/2 inch thick and 2 inches long.
Season generously with salt and pepper.
Let the meat stand for 2 hours in a cool place or in the refrigerator.
Make a roux by blending 2 tablespoons butter with 1 tablespoon flour over a gentle heat until the mixture bubble and is smooth.Gradually stir in 2 cups beef stock.
Cook until the mixture begins to thicken.
Boil for 2 minutes.
Stir in 2 teaspoons prepared mustard.
Stir in 4 tablespoons thick sour cream.. stirring constantly.