Bring a large pot of salted water to a boil. Add the wide egg noodles and cook according to package directions, typically about 7-9 minutes, until perfectly al dente. Drain the pasta immediately and set it aside, reserving about 1/2 cup of the starchy pasta water for sauce consistency if needed.

In a large, deep skillet or Dutch oven, melt the butter (or heat the oil) over medium-high heat. Add the ground beef. Break it up with a spoon and sear it aggressively until deeply browned, about 5-7 minutes. Season with salt and pepper. Drain off any excess grease, leaving about 1 tablespoon for the vegetables.

Reduce the heat to medium. Add the diced onion and cook until softened and translucent, about 3-4 minutes. Stir in the sliced mushrooms and cook for another 5-7 minutes until the mushrooms release their moisture and become deeply browned and tender.

Push the meat and vegetables to one side and add the minced garlic. Cook for 1 minute until fragrant. Sprinkle the 2 tablespoons of flour over the mixture. Stir well, coating everything, and cook for 1 minute to remove the raw flour taste.

Pour in the beef broth, scraping up the browned bits from the bottom of the pan. Stir until smooth and bring to a gentle simmer. Add the Worcestershire sauce and Dijon mustard (if using). Taste and adjust seasoning with salt and pepper. Let simmer for 3-5 minutes until slightly thickened.

Remove the skillet from the heat. Stir in the sour cream (or Greek yogurt) until the sauce is creamy and smooth. If too thick, thin with reserved pasta water.

Add the drained egg noodles to the skillet with the sauce. Toss gently to coat. Serve immediately, garnished with fresh parsley.