Stir in the melted butter, sugar, salt, and pepper. Top with thyme and bay leaf, COVER, and cook on LOW for about 8 hours or until onions are browned and caramelized. Stir halfway through.
Once onions are caramelized, stir in the beef broth and wine. Taste for salt and pepper and season accordingly.
Cover and cook on LOW for 4 hours, or on HIGH for 2 hours, or until heated through.
Remove the lid and discard the thyme and bay leaf. Ladle the soup into ovenproof bowls and transfer the bowls to a large baking sheet.
Top the soup in the bowls with a slice of French bread and shredded cheese.
Turn the oven-broiler to High.
Broil the soups in the oven until the cheese is melted, about 2 to 3 minutes.
Remove from oven and serve.
Notes
For a lighter option, omit the bread and cheese.
For a quicker version, start the soup on the stovetop. Mix onions, butter, sugar, salt, and pepper in a big skillet. Cook until they're brown and caramelized, around 25 minutes. Then, shift the onions into the slow cooker, and add beef broth, wine, thyme, salt, and pepper. Let it cook for 6 hours on LOW. When done, pour it into oven-safe bowls, add a baguette slice to each bowl, sprinkle shredded cheese on top, and broil until the cheese melts.
Keep a close watch on those onions. It's a narrow line between lovely golden caramelized onions and scorched ones. Once they're caramelized, pull the pan off the heat to avoid a bitter soup.
Use croutons: If you're out of French bread, swap it for your favorite croutons to get that same crunchy bite.