Season the steak with salt and toss in about half (2 tablespoons) of the flour. Set aside.

Put ½ cup of the sour cream (120g) in a small bowl. Set aside. Combine the remaining 1 cup (240g) of the sour cream in a small bowl with the remaining flour. Stir well to combine. Set aside.

Sear half of the meat in 1 tablespoon (14g) the butter over medium heat until it forms a dark golden crust, about 4 minutes. Do not crowd the pan. Remove the meat to a plate as it is done browning. Repeat with the remaining meat and 1 more tablespoon of butter. Set the seared meat aside.

Add the remaining 2 tablespoons (28g) butter to the thick fond in the pan. Add the onions and mushrooms to the pan. Turn up the heat to medium-high. Season with salt. Let the mushrooms sit until they release their liquid, then stir to scrape up the fond from the bottom of the pan. Let the mushroom liquid reduce by half, about 4 minutes.

Add the wine into the mushroom mixture and simmer until the liquid is reduced by half, about 5 minutes.

Add the reserved meat and its juices back to the pan along with the beef stock and mustard. Stir to combine.

Whisk in the reserved sour cream/flour mixture. Bring to a boil, reduce heat, and simmer, stirring occasionally, until the sauce is thickened and the meat is very tender, about 45 minutes.

Add the remaining ½ cup sour cream and paprika into the sauce. Taste for seasoning, adding more salt if needed. Serve over cooked egg noodles.