1. In a large skillet, over medium heat, add the butter. Once melted, add the mushrooms, occasionally stirring, and cook for about 5 minutes until slightly softened with a light sear. Place them on a plate and set them aside.
2. Add the ground beef and break it up into crumbles, occasionally stirring until cooked through with no pink left, 8-10 minutes.
3. Add the onions and stir them into the beef. Cook until softened and translucent, 8-10 minutes, occasionally stirring.
4. Add the garlic until fragrant, 30 seconds. Add the flour and stir it in until everything is coated, and cook for 1 minute.
5. Slowly stream in the beef stock while constantly stirring. Add the heavy cream, Worcestershire sauce, salt, and pepper, and stir to combine.
6. Add the mushrooms back to the skillet and stir them in. Bring to a simmer and simmer for 5 minutes.
7. Take them off the heat and stir in the sour cream until combined. Taste and adjust seasoning if necessary. Serve immediately over egg noodles with fresh parsley
if using.
See notes below.