Pat the beef strips dry with paper towels and season generously with salt and pepper.
Heat oil in a large skillet over high heat. Working in batches, sear the beef until browned (about 1 minute per side). Remove and set aside.
In the same pan, add butter and sauté mushrooms until golden brown (about 5 minutes). Remove and set aside.
Reduce heat to medium and cook onions until soft and translucent (about 5 minutes). Add garlic and cook for another minute.
Sprinkle flour over the onions and stir for 1-2 minutes to cook out the raw flour taste.
Gradually whisk in beef broth, stirring constantly to prevent lumps. Simmer until sauce begins to thicken.
Stir in Dijon mustard, then gradually add room temperature sour cream, whisking constantly.
Return beef and mushrooms to the pan, heating through but not boiling (to prevent curdling).
Serve hot over cooked egg noodles, garnished with fresh parsley.