Place sliced onions, butter, and sugar in the slow cooker and toss to combine.
Cook on high for 1 hour uncovered, stirring once halfway through to help onions begin caramelizing.
Add beef broth, white wine, bay leaves, thyme, salt, and pepper, then stir well.
Cover and cook on low for 6-8 hours until onions are deeply caramelized and golden brown.
Remove bay leaves and ladle soup into oven-safe bowls, top each with a slice of toasted French bread and generous amount of Gruyère cheese.
Place bowls on a baking sheet and broil for 3-5 minutes until cheese is melted, bubbly, and golden brown.