Pat the beef chuck roast dry with a paper towel, then season all sides with flour, salt, and black pepper. Heat vegetable oil in a skillet and brown the beef for 4-5 minutes on each side. Transfer to the slow cooker.

In the same skillet, add chopped yellow onion and sauté until caramelized, about 5-7 minutes. Add minced garlic and cook for another minute, then transfer to the slow cooker.

Pour beef broth over the beef and onion mixture in the slow cooker. If using mushrooms, add them now with the dried thyme. Stir well to combine.

Cover and set the slow cooker to HIGH. Cook for about 4 hours until the beef is tender.

Add wide egg noodles and dried parsley to the slow cooker. Reduce heat to LOW, cover, and cook for another 30 minutes, or until noodles are tender. Allow to rest for 5-10 minutes before serving.