1. Cut 2 pounds tender beef into narrow strips 1/2 inch thick and 2 inches long.
2. Season generously with salt and pepper.
3. Let the meat stand for 2 hours in a cool place or in the refrigerator.
4. 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.
5. Cook until the mixture begins to thicken.
6. Boil for 2 minutes.
7. Stir in 2 teaspoons prepared mustard.
8. Stir in 4 tablespoons thick sour cream.. stirring constantly.
9. Simmer gently without boiling.
10. Brown the pieces of meat in 3 tablespoons butter.
11. Sauté 3 cups mushrooms and 1 onion (both firmly sliced), lightly in butter.
12. Cover the saucepan.
13. Put it over hot water for 20 minute.
14. Serve hot.

List of ingredient required:

• 2 pounds tender beef.
• Salt.
• Pepper.
• Butter.
• 1 tablespoon flour.
• 2 cups beef stock.
• 2 teaspoons prepared mustard.
• 4 tablespoons thick sour cream.
• 3 cup[s mushrooms, firmly sliced.
• 1 onion, firmly sliced.