Melt butter in the pan over medium heat.
Sautee mushrooms until tender, about 3-4 minutes.
Add onion. Sautee for about 5 minutes until tender. Add garlic and sautee until fragrant, about 2 minutes.
Add ground beef and cook until brown. Be sure to "crumble" the beef as it cooks. Do not drain any excess fat if using lean ground beef. You want a bit of fat left in the pan.
Stir in flour until it is browned.
Stir in beef broth, thyme, Worcestershire, and egg noodles. Let the mixture simmer until the noodles are cooked, about 10 minutes. Add salt and pepper, to taste.
Once noodles are done, stir in sour cream until heated, about 2 minutes. Serve immediately.