Prepare the Beef: Slice flank steak across the grain and season with homemade rosemary salt and black pepper.
Dredge the Beef: Mix flour, corn starch, onion powder, and garlic powder. Dredge the seasoned beef in the flour mixture.
Fry the Beef: Fry the beef in beef fat or high smoke point oil until golden brown, then set aside.
Cook Mushrooms and Onions: In the same pan, cook mushrooms until golden, then set aside. Add more fat or oil and cook onions until golden brown. Add garlic and thyme and cook for 2 minutes.
Deglaze and Reduce: Deglaze the pan with dry white wine, add Worcestershire sauce, beef stock, and bring to a simmer. Reduce by 80%.
Finish the Sauce: Add Dijon mustard, then gradually add heavy cream to desired color and consistency. Work in cubes of cold unsalted butter for richness.
Combine Beef and Sauce: Add the cooked beef back into the sauce and warm through. Adjust seasoning with salt.
Serve: Serve over egg noodles, garnished with a dollop of sour cream and fresh parsley leaves.