Add sliced Sunions and cook until reduced by half and caramelized -- approximately 40 minutes, stirring occasionally. Do not let them burn. Add garlic, thyme, mustard, and Worcestershire sauce and stir. Sauté for 1 minute. Deglaze with white wine, making sure to scrape up all the bits on the bottom of the pan -- this is all the great flavor! Stir in beef stock and bay leaf and cover. Simmer 10-15 minutes. Remove bay leaf.
Distribute soup into individual crocks. Place toasted baguette slices on top, followed by a generous pile of Gruyère cheese.
Preheat broiler to HIGH. Place crocks under broiler until the cheese is bubbling and slightly browned, about 2-4 minutes. (Make sure to keep an eye on it!)