Season the meat with salt and pepper to taste. Place in the bottom of a slow cooker. Add the mushrooms, onion, and bay leaf.
In a large bowl or measuring cup, whisk together the condensed soup, beef broth, wine, tomato paste, Worcestershire sauce, garlic, thyme, and ½ teaspoon of pepper.
Pour the soup mixture into the slow cooker.
Cover and cook until the beef is tender, about 8-9 hours on LOW or 4-5 hours on HIGH.
Discard the bay leaf. Using a large spoon, skim fat from the surface of the sauce.
In a medium bowl, whisk together ½ cup of the warm sauce with the sour cream, mustard, and dill. This tempers the sour cream so that it doesn’t break when it hits the hot sauce. Stir the mixture back into the slow cooker.
Taste and season with salt and pepper, as needed.
Serve beef and gravy over cooked egg noodles, mashed potatoes, or rice, and garnish with fresh parsley, if desired.