Cook onions in butter for 20–30 minutes until softened and golden.
Season and brown the beef.
Add onions and beef to the slow cooker.
Add garlic, broth, sherry, Worcestershire sauce, thyme, bay leaf, salt, and pepper.
Cook on LOW for 7–8 hours.
Shred the beef and return it to the soup.
Top servings with toast and cheese, then broil until bubbling.