Caramelize the Onions: In a large pot, heat olive oil over medium heat. Add sliced onions and salt, stirring occasionally. Cook for 30–40 minutes until golden brown.
Add Flavor: Stir in garlic, thyme, black pepper, and balsamic vinegar. Cook for another minute.
Simmer the Soup: Pour in broth. Bring to a boil, then reduce heat and let it simmer for 20 minutes.
Prepare the Bread: Toast bread slices until golden.
Assemble and Broil: Ladle the soup into oven-safe bowls, place a toasted bread slice on top, and sprinkle with cheese. Broil for 2–3 minutes until cheese melts and bubbles.
Serve and Enjoy: Garnish with extra thyme and serve hot!