Beef Stroganoff

Cut the meat into 1/2 inch thick slices. Place between 2 sheets of waxed paper. Pound with a mallet or a heavy plate until meat is 1/4 inch thick. Be careful not to tear meat. Cut meat into 2x 1/2 inch slices. Place slices in one layer on a large platter. Sprinkle with salt and pepper and let stand at room temperature for 15 minutes. Heat 2 Tbsp. of the butter in a deep, heavy frying pan or shallow saucepan large enough to hold all the ingredients. Add the sliced onion and cook over medium heat, stirring constantly, for about 5 minutes or until beginning to soften. Add meat to the pan. Cook for 3 minutes, turning beef slices constantly so that they are evenly browned on all sides. Stir in flour and mustard and cook, stirring constantly, for 1 more minute. Add the 1/2 cup of wine and optional tomato paste. Reduce heat to low. cover pan and simmer for 5 to 10 minute, sitting frequently. In another frying pan,m heat the remaining 2 tablespoons of butter. Add the minced onion and the mushrooms. Cook over medium heat for 2 minutes, add the 2 tablespoons of wine and cook for 2 more minutes: mushrooms should still be firm. Add mushrooms and their liquid to pan containing the meat. Check seasoning and stir in sour cream. Over lowest possible heat, simmer for about 5 minutes to heat dish through. Do not boil. Serve over a bed of hot cooked rice or kasha, with a green vegetable on the side.