Season the beef with salt and pepper. Place it in the slow cooker along with the mushrooms, onion, carrots, and garlic.
In a bowl, whisk together the Guinness stout, beef broth, tomato paste, Worcestershire sauce, thyme, and flour until smooth. Pour the mixture over the meat and vegetables.
Add the bay leaves, then cover and cook on low for 8 hours or until the beef is very tender.
Remove the bay leaves, taste the stew, and adjust salt and pepper if needed before serving.