Boil a large pot of salted water and cook the egg noodles according to package instructions. Drain and set aside.

Heat a large heavy-bottomed skillet over medium-high heat. Add the olive oil and 1 tablespoon of butter. Sear the beef slices in batches for 1 to 2 minutes per side until deeply browned. Remove the beef and set aside.

Lower the heat to medium. Add the remaining 3 tablespoons of butter to the skillet. Sauté the chopped onions for 3 to 4 minutes until softened, then add the sliced mushrooms. Cook until the mushrooms are golden brown and have released their moisture, about 5 to 7 minutes. Add the minced garlic and cook for 1 minute more.

Sprinkle the flour over the mushroom and onion mixture, stirring constantly for 1 minute to cook out the raw flour taste.

Gradually pour in the beef broth, scraping up any browned bits from the bottom of the pan. Stir in the Dijon mustard. Bring the mixture to a simmer and let it thicken for 3 to 5 minutes.

Reduce the heat to low. Stir in the sour cream until completely smooth and heated through. Do not let the sauce boil, or the sour cream may curdle.

Return the cooked beef and any resting juices to the skillet, stirring gently to combine. Season with salt and pepper to taste.

Serve the creamy beef stroganoff warm over the prepared egg noodles, garnished with freshly chopped parsley.