Prepare the beef: In a medium bowl, combine the thinly sliced beef strips with 15g (2 tablespoons) of all-purpose flour, 2.5g (1/2 teaspoon) of salt, and 1g (1/4 teaspoon) of black pepper. Toss gently to coat the beef evenly. This flour coating will help thicken the sauce later. Set aside.

Cook the noodles: Bring a large pot of salted water to a rolling boil. Add the egg noodles and cook according to package directions until al dente, typically 7-10 minutes. Drain the noodles well, reserving about 60ml (1/4 cup) of the pasta water. Return the drained noodles to the pot, toss with 15g (2 tablespoons) of butter and a pinch of salt and pepper. Cover to keep warm.

Sear the beef: Heat 15ml (1 tablespoon) of olive oil and 15g (1 tablespoon) of butter in a large skillet or Dutch oven over medium-high heat until the butter is melted and shimmering. Add half of the floured beef strips in a single layer, ensuring not to overcrowd the pan. Sear for 1-2 minutes per side until nicely browned. Do not cook through. Remove the seared beef to a clean plate and repeat with the remaining beef. Set aside.

Sauté the aromatics and mushrooms: Reduce the heat to medium. Add the remaining 30g (2 tablespoons) of butter to the same skillet. Add the finely chopped yellow onion and cook, stirring occasionally, until softened and translucent, about 5-7 minutes. Add the minced garlic and sliced mushrooms. Cook, stirring frequently, until the mushrooms have released their liquid and started to brown, about 6-8 minutes. This browning develops their flavor.

Deglaze and build the sauce: Sprinkle the remaining 15g (2 tablespoons) of flour over the cooked onions and mushrooms. Stir well and cook for 1 minute to toast the flour, which helps prevent a raw flour taste. Gradually whisk in the beef broth, scraping up any browned bits from the bottom of the pan. Stir in the Worcestershire sauce and Dijon mustard. Bring the mixture to a simmer and cook, stirring constantly, until the sauce begins to thicken, about 3-5 minutes.

Combine and finish: Return the seared beef strips (and any accumulated juices) to the skillet with the sauce. Stir to combine and let it simmer gently for about 5 minutes, or until the beef is cooked through and tender. Be careful not to overcook the beef, as it can become tough. Stir in the sour cream until fully incorporated and the sauce is smooth and creamy. Do not boil the sauce after adding the sour cream, as it can curdle. Heat through gently over low heat.

Serve: Taste and adjust seasoning with salt and pepper if needed. Ladle the creamy Beef Stroganoff generously over the buttered egg noodles. Garnish with fresh chopped parsley for a burst of color and freshness.