Pat the beef dry with paper towels and slice it into thin strips against the grain. Season generously with salt and black pepper, then lightly coat the beef with flour. This helps create a beautiful sear while also slightly thickening the sauce later.

Heat the olive oil in a large skillet over medium-high heat. Add the beef in small batches and sear for about 1 to 2 minutes per side until nicely browned. Avoid overcrowding the pan to ensure proper caramelization. Transfer the beef to a plate and set it aside.

Reduce the heat to medium and melt the butter in the same skillet. Add the sliced onions and cook until softened, about 4 minutes. Stir in the garlic and mushrooms, cooking for another 6 to 8 minutes until the mushrooms release their moisture and become golden brown.

Pour the beef broth into the skillet, scraping up any browned bits from the bottom of the pan. Stir in the Worcestershire sauce, Dijon mustard, and paprika. Allow the sauce to simmer gently for about 5 minutes. If you’d like a thicker sauce, whisk the cornstarch with a little cold water and stir it into the skillet.

Reduce the heat to low and stir the sour cream into the sauce until smooth and creamy. Return the seared beef and any accumulated juices to the skillet, stirring gently until the beef is warmed through. Avoid boiling the sauce after adding the sour cream, as it can cause it to separate.

Divide the cooked egg noodles among serving plates and spoon the creamy beef stroganoff generously over the top. Finish with a sprinkle of freshly chopped parsley and a light dusting of black pepper for added freshness and color. Serve immediately while hot with crusty bread or a simple green salad for a complete and comforting meal.