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 aside.
Add the ground beef and break it up into crumbles, occasionally stirring until cooked through with no pink left, 8-10 minutes.
Then add the onions and stir them into the beef. Cook until softened and translucent, 8-10 minutes, occasionally stirring.
Add in the garlic and stir until fragrant, about 30 seconds. Add in the flour and stir until everything is coated and cook for 1 minute to get the flour taste out.
Slowly stream in the beef stock while constantly stirring. Add the heavy cream, worcestershire sauce, salt, and pepper, and stir to combine.
Add the mushrooms back to the skillet and stir them in. Bring to a simmer and simmer for 5 minutes.
Take 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.