Thinly slice the onions from root to tip so they hold their shape as they cook. Try to keep the slices even for consistent caramelization. Yes, you may shed a few tears, but I always say they’re worth it.

In a large Dutch oven, melt the butter with the olive oil over medium heat. Add the sliced onions, salt, and pepper. Stir to coat them well. Cook for about 10 minutes until they soften and become translucent, stirring occasionally.

Reduce the heat to medium-low and continue cooking for 30–40 minutes, stirring every few minutes. If using, sprinkle in the sugar to encourage browning. The onions should slowly turn a deep golden brown and smell sweet and nutty. If they stick slightly to the bottom, that’s flavor—just stir and scrape gently.

Stir in the minced garlic and cook for 1 minute until fragrant. Pour in the white wine and scrape up any browned bits from the bottom of the pot. Let it simmer for 2–3 minutes until slightly reduced.

Sprinkle in the flour if using and stir well. Add the beef broth, chicken broth, thyme, and bay leaf. Bring to a gentle simmer and cook uncovered for 20–30 minutes. The aroma at this stage is absolutely irresistible—rich, savory, and comforting.

While the soup simmers, preheat your oven to 400°F (200°C). Arrange baguette slices on a baking sheet and toast for 8–10 minutes until golden and crisp. They should be firm enough to hold up in the soup.

Preheat your broiler. Ladle the hot soup into oven-safe bowls. Place a toasted baguette slice on top of each bowl and generously sprinkle with Gruyère and a bit of mozzarella. Set the bowls on a baking sheet and broil for 2–4 minutes until the cheese is melted, bubbly, and golden brown.

Watch carefully—cheese can go from perfect to burnt in seconds!

Carefully remove from the oven (the bowls will be very hot). Let them sit for a minute before serving. Break through that cheesy crust with your spoon and enjoy every single spoonful of your homemade French Onion Soup.