Beef Stroganoff (From The "Old" Russian Tea Room) - cooking recipe

Ingredients

2 lbs lean boneless sirloin (trimmed of fat and gristle) or 2 lbs bottom round steaks, in one piece (trimmed of fat and gristle)
2 teaspoons salt
1/2 teaspoon fresh ground pepper
4 tablespoons butter
1 medium onion, thinly sliced
1 tablespoon flour
1 teaspoon mustard powder (OR 1 Tablespoon prepared Dijon style mustard)
1/2 cup dry white wine
2 teaspoons tomato paste (optional)
1 tablespoon minced onion
1/2 lb mushroom, thinly sliced
2 tablespoons dry white wine
1 cup sour cream, warmed

Preparation

Cut meat into 1/2 inch thick slices. Place between sheets of waxed paper and pound until its 1/4 inch thick (be careful not to tear meat). Cut pounded meat into 2x1/2 inch slices. Sprinkle meat with salt and pepper, let stand for 15 minutes.
Heat 2 TBS butter in a frying pan large enough to hold everything. Add sliced onion, cook over medium heat for 5 minutes.
Add meat to pan and cook for 3 minutes, turning meat to brown evenly.
Stir in flour and mustard, and cook 1 minute more. Add 1/2 cup wine and optional tomato paste. Reduce heat to low, cover pan and simmer for 5-10 minutes, stirring frequently.
In another frying pan, heat 2 TBS butter. Add minced onion and mushrooms. Cook over medium heat 2 minutes. Add 2 TBS wine and cook 2 minutes more. Add mushroom mixture to meat mixture. Check seasonings and add warmed sour cream.
Over lowest possible heat, simmer for 5 minutes to heat through. Do NOT let it boil.
Serve over hot rice with a green vegetable on the side.