This recipe is from the "old" Russian Tea Room Restaurant in NYC (before the original owner's daughter took it over.) Authentic in flavor and preparation (no short cuts) and was always served over RICE (not noodles) at the restaurant.
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.