Season the steak with salt and pepper, then slice it thinly against the grain.
Heat olive oil in a large skillet over medium-high, sear the steak in batches until browned, and set aside.
Add butter to the same skillet and sauté diced onion and sliced mushrooms until softened, about 5 minutes.
Stir in chopped garlic, cook for one minute, then sprinkle flour over the vegetables and mix well.
Pour in beef broth while scraping the bottom of the pan, then add Dijon mustard, Worcestershire sauce, and any juices from the steak.
Let it simmer for a few minutes, reduce the heat, and stir in the sour cream until the mixture is smooth.
Return the steak to the skillet and cook a few minutes, just until heated through.
Serve the beef stroganoff over cooked egg noodles and sprinkle chopped parsley on top.