Instructions

1. Melt 4 tablespoons butter in a large, heavy-bottomed pot over medium heat. Add thinly sliced onions and cook gently, stirring frequently, for about 40-50 minutes until onions are deeply caramelized and golden brown (do not rush this step—slow caramelization is key).
2. Sprinkle with sugar and continue cooking for 2-3 more minutes to deepen the caramelization. Season with salt and pepper.
3. Optional: Sprinkle flour over the onions, stir well, and cook 1-2 minutes to create a light roux for a silkier soup.
4. Add white wine, scraping the bottom of the pot to release browned bits. Let simmer 2-3 minutes until wine is mostly evaporated.
5. Add hot stock, bay leaves, and thyme sprigs. Bring to a boil, then reduce to a simmer. Cook uncovered for 15-20 minutes to let flavors meld. Taste and adjust seasoning with salt and pepper.
6. While soup simmers, preheat oven to 400°F / 200°C. Toast bread slices on a baking sheet with 2 tablespoons butter until lightly golden on both sides, about 3-4 minutes.
7. Rub toasted bread with a cut garlic clove if desired. Top each slice generously with grated Gruyère cheese.
8. Discard bay leaves and thyme sprigs from soup. Ladle soup into oven-safe bowls or a large oven-safe pot. Place toasted cheese bread slices on top of each bowl (or distribute across the top of a larger pot).
9. Bake 8-10 minutes until cheese is melted, bubbly, and golden brown. Let cool slightly before serving, as it will be very hot. Serve immediately.