Heat the olive oil in a large skillet over medium-high heat. Add the ground beef and cook, breaking it apart, until browned. Drain off any excess grease.
Add the chopped onion and sliced mushrooms to the skillet with the beef. Cook until the onions soften and the mushrooms release their liquid, about 5 minutes.
Stir in the minced garlic, thyme, salt, and pepper. Cook for 1 minute until fragrant.
Sprinkle the flour over the beef and vegetable mixture. Stir well to coat everything.
Pour in the beef broth and Worcestershire sauce. Bring the mixture to a simmer, scraping up any browned bits from the bottom of the pan.
Add the egg noodles to the skillet, ensuring they are mostly submerged in the liquid. Cover the skillet and reduce the heat to medium-low. Cook for 8 to 10 minutes, or until the noodles are tender and most of the liquid is absorbed. Stir occasionally to prevent sticking.
Remove the skillet from the heat. Let it cool for 2 minutes. Stir in the room temperature sour cream until the sauce is smooth and creamy. Do not boil after adding sour cream.
Serve immediately, garnished with fresh parsley.
Notes
Use room temperature sour cream to prevent the sauce from curdling when you stir it in at the end.
If the sauce seems too thick after adding the noodles, add a splash more broth or water until you reach your desired consistency.
For a richer flavor, substitute half of the beef broth with dry red wine and let it cook down for 2 minutes before adding the flour.