My very favorite stroganoff. The original recipe came from Ruth Mellinkoff's "The Uncommon Cook Book". The original recipe uses beef tenderloin, but to economize I use NY steak or beef sirloin (choice cut) .
Saute mushrooms and onion in the 3 T butter and add salt and pepper. Cook 3 minutes and add the 1/2 C Sherry wine and stir. Slowly mix in the sour cream using a wire whisk and add the paprika, and lemon juice. Set aside.
In a small saucepan, melt the 2 T butter with 2 T flour. Add the beef stock over low heat and stir until sauce is very thick. Add this into the sour cream mixture.
Cut the meat into slices 2" by 1/4" and then season with some salt and pepper. Sear them quickly in the 1 T oil until browned on both sides and remove from the pan. Don't do too many slices at a time (it won't sizzle). Continue with remaining meat slices and remove from pan. Drain fat from the skillet and add 1/4 C sherry wine to the pan and deglaze.
Stir in tomato paste, sour cream mixture, and meat. Taste and add more salt, lemon juice or paprika as needed.