Bring a large pot of salted water to a boil and cook the egg noodles according to package instructions until al dente. Drain well and set aside.

Season the sliced steak generously with salt and black pepper. Heat the olive oil in a large skillet over medium-high heat. Add the beef in a single layer and sear until nicely browned on the outside, about 1-2 minutes per side. Remove the beef from the skillet and set aside on a plate.

Lower the heat to medium and add the butter to the same skillet. Add the chopped onion and sliced mushrooms, sautéing until the mushrooms are browned and the onions are soft, about 5-7 minutes. Stir in the minced garlic and cook until fragrant, about 30 seconds.

Sprinkle the flour over the mushroom mixture and stir well to coat, cooking for 1 minute to remove the raw flour taste. Gradually pour in the beef broth, scraping up any browned bits from the bottom of the pan. Whisk in the Dijon mustard and Worcestershire sauce.

Bring the sauce to a gentle simmer. Allow it to cook and thicken slightly, about 3-5 minutes, until it coats the back of a spoon.

Reduce the heat to low. Stir in the sour cream until smooth and creamy, making sure the sauce does not boil to prevent curdling. Return the seared beef and any resting juices to the skillet, stirring gently to warm through for 1-2 minutes.

Taste the sauce and adjust the seasoning with additional salt or pepper if needed. Spoon the creamy stroganoff over the warm egg noodles, garnish with fresh chopped parsley, and serve immediately.