Season the beef strips generously with salt and pepper; set aside
In a large skillet over high heat, add oil. When it gets hot, working in batches, add seasoned beef and cook for 1 1/2 minutes or until slightly brown, stirring constantly.
Using a slotted spoon, transfer the cooked strips to a bowl while cooking the remaining beef; set aside.
Reduce the heat to medium and add butter.
When melted, add onions and mushrooms; sauté until brown, about 5 minutes. Add garlic and stir for 30 seconds. Stir in flour and cook for 2 minutes until incorporated.
Pour in wine and beef stock, scraping the bottom of the pan to release any browned bits. Bring to a simmer and cook for 4-5 minutes or until sauce thickens.
Add cooking sherry, Worcestershire sauce and Dijon mustard; stir until well mixed.
Return the beef strips to skillet and bring to a simmer. Cover, reduce the heat to low and cook for 45 minutes or until the beef is tender and the sauce is thick, stirring a couple times.
Meanwhile, in a bowl, whisk sour cream and cream cheese with ¼ cup of simmering sauce.
5 minutes before the cooking time is over, stir in the sour cream mixture to the skillet.
When the cooking time is up, sprinkle on garlic salt and adjust seasoning if needed. If the sauce is not thick enough, add the cornstarch mixture.
Ladle the beef mixture over egg noodles with a dollop of sour cream; sprinkle parsley on top.