Slice sirloin into ¼-inch strips and season with salt, pepper, and paprika.
Heat oil in a Dutch oven. Sear beef in batches until golden. Remove and set aside.
Melt butter and sauté mushrooms until browned, then add onions until softened.
Stir in flour, cook until lightly golden.
Pour in white wine, scrape up browned bits, and add beef stock. Simmer until thickened.
Remove from heat and whisk in sour cream, Worcestershire, and Dijon mustard.
Return beef to pot, coat in sauce, and serve over buttered egg noodles with parsley.
