Place the beef stew meat, sliced mushrooms, and sliced onions into the slow cooker.
Pour in the beef broth, then add the dried thyme, salt, and black pepper.
Stir everything together, cover, and cook on low for 8 hours until the beef is very tender.
In a small bowl, whisk the heavy cream and cornstarch until smooth.
Stir the cream mixture into the slow cooker, cover again, and cook on high for 30 minutes to thicken.
Ladle the stew into bowls and top with chopped fresh parsley before serving.