French Onion Soup is such a classic and SO easy to make! Onions are slowly caramelized until soft and sweet, simmered with beef broth, thyme, and just a touch of sugar. It's topped with a crispy crouton, loads of shredded gruyere cheese and broiled into a crispy, cheesy oblivion. So good!
Heat a large heavy-bottomed pot to a medium heat. Add the butter. Once the butter melts, add the onions, garlic cloves, salt, and sugar. Stir to coat. Cook the onions for about 5 minutes until they start to soften. Once they soften a bit, turn the heat down to medium-low and cook another 10 minutes. When they start to get super soft, turn the heat down again to low. Continue to cook the onions, while stirring every few minutes until the onions are golden brown and caramelized. It should take another 15 minutes or so.
Stir the flour into the onions. Cook for 1 minute. Slowly stir in the broth and add the thyme. Bring the mixture up to a boil and let it simmer. Simmer until slightly thicker, about 4-5 minutes.
While the onions cook. Add bread pieces to a sheet pan. Bake in the oven until toasted, about 5 minutes.
Pour the soup into four oven-proof French Onion Soup bowls, Once the soup and croutons are ready to go, put one crouton on top of the soup. Cover with cheese. Place the soups on a large sheet pan and broil until the cheese has melted and started to turn brown and crispy, about 2-3 minutes. Serve immediately.