Heat a large deep skillet or Dutch oven over medium-high heat. Add the ground beef and cook for 5–7 minutes, breaking it up as it browns; drain excess fat if necessary.

Add the diced onion and sliced mushrooms. Cook for 4–5 minutes, stirring occasionally, until the onion softens and the mushrooms release most of their moisture.

Stir in the minced garlic and cook for about 30 seconds, just until fragrant.

Sprinkle the flour over the beef and vegetables. Stir for 1 minute so the flour coats everything evenly.

Gradually pour in the beef broth while stirring to prevent lumps. Add the Worcestershire sauce, Dijon mustard, kosher salt, and black pepper.

Stir in the uncooked wide egg noodles. Bring the mixture to a gentle boil, then reduce the heat, cover, and simmer for 8–10 minutes, stirring once or twice so the noodles do not stick.

Remove the pan from the heat and let it stand for 1–2 minutes. Stir in the sour cream, taste, and adjust the seasoning.

Garnish with chopped fresh parsley, if using, and serve immediately.