🧅 Caramelize the Onions
- Melt the butter in a large Dutch oven over medium heat.
- Add the onions, sugar, and salt and cook, stirring often, for 35 to 45 minutes until deeply caramelized.
🍷 Deglaze the Pot
- Stir in the garlic and cook for 30 seconds.
- Pour in the wine and scrape up any browned bits.
- Sprinkle the flour over the onions and stir for 1 minute.
🍲 Simmer the Soup
- Add the beef broth, thyme, and bay leaf.
- Simmer uncovered for 25 minutes.
- Stir in the balsamic vinegar and season with pepper.
🧀 Broil the Tops
- Ladle the soup into oven-safe bowls and top each with 2 baguette rounds.
- Cover generously with Gruyère and broil for 3 to 4 minutes until bubbling and golden.
- Serve immediately.