Melt the butter in a large pan over medium-high heat until sizzling. Add sliced mushrooms and onions. Cook until just tender, about 2 minutes.
Add the garlic and thyme to the pan and cook until fragrant, about 1 more minute. Transfer everything to a plate and set aside.
Using the same pan, brown the ground beef until no pink remains. Drain as needed. Stir the flour into the browned beef.
Over medium heat, whisk in the broth and worcestershire sauce until no lumps of flour remain. Bring liquid to a low boil. Return the mushrooms, onions garlic and thyme to the pan.
Add the pasta and stir to cover. Cover pot and boil for 9 - 10 minutes until pasta is tender.
Remove pan from the heat. Stir in sour cream until completely combined and sauce is smooth.
Sprinkle with salt and pepper to taste. Garnish with parsley and serve.