1. Combine onions, butter, thyme, bay leaf, brown sugar, 1/2 teaspoon salt, and 1 teaspoon pepper in a large Dutch oven over medium heat. Cover and cook, stirring once or twice, until onions have softened and liquid is bubbling vigorously, about 15 minutes.
2. Uncover and cook, stirring occasionally, until liquid has evaporated and onions begin to sizzle, about 15 minutes. Reduce heat to low and cook, stirring frequently, until onions are silky and deeply browned and a coating forms on bottom of pot, about 40 minutes, adjusting heat as necessary to prevent burning.
3. Increase heat to medium-high, add 1/2 cup vermouth and the soy sauce, and cook, scraping up browned bits from bottom of pot, until liquid has evaporated, about 2 minutes. Add both broths and bring to a boil over high heat, then reduce heat to medium and simmer 20 minutes. Remove from heat and stir in remaining 1/4 cup vermouth. Discard thyme sprigs and bay leaf and season to taste with salt and pepper.
4. Place a rack in upper-middle of oven and heat broiler. Line a baking sheet with foil and add baguette slices. Broil until slices are dry and crisp, about 1 minute per side; set aside. Place six broiler-safe soup bowls or crocks on baking sheet, then ladle soup into bowls, leaving 1/2" of space at the top. Top each bowl with two baguette slices (trim slices to fit if needed) and sprinkle all over with cheese. Carefully transfer baking sheet to oven and broil until cheese is bubbling and browned, 2 to 5 minutes. Serve immediately.
Source: Hannaford fresh Magazine, January - February 2020