In a large skillet, melt the butter over medium heat. Add the sliced onions, salt, and sugar. Cook, stirring occasionally, for about 15-20 minutes until the onions are caramelized and golden brown.
Transfer the caramelized onions to the crockpot. Add the minced garlic, beef broth, white wine, thyme, bay leaf, Worcestershire sauce, and black pepper. Stir to combine.
Cover and cook on low for 6-8 hours or on high for 3-4 hours.
When ready to serve, preheat your oven broiler. Ladle the soup into oven-safe bowls, top each with a slice of baguette, and sprinkle generously with Gruyère cheese.
Place the bowls on a baking sheet and broil for 2-3 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let cool slightly before serving.
Notes
For a vegetarian version, substitute beef broth with vegetable broth and omit the Worcestershire sauce or use a vegetarian alternative.
Add a splash of sherry or balsamic vinegar for an extra depth of flavor before serving.