Instructions:
Start the Onions: Place the onions, butter, sugar, and salt in the bottom of a 6-quart slow cooker. Cover and cook on high for 7 hours, or until the onions are soft and golden brown.
Add Broth & Seasonings: Once the onions are caramelized, add the beef broth, sherry, thyme, and pepper. Cover and cook on high for another hour.
Prepare to Serve: Ladle the soup into oven-safe mugs or bowls. Top each with sliced bread (or croutons) and a generous layer of Gruyere cheese.
Melt the Cheese: Place the mugs under the broiler for a few minutes, until the cheese is melted and golden brown. You can cook for an additional 15 minutes if you like your cheese extra melted!
Pro Tip:
