Step 1

Melt the butter in a large pot or Dutch oven over medium heat. Add the olive oil, onions and sugar. Cook, stirring occasionally, until onions are soft, lightly browned and caramelized, about 30 minutes.

Step 2

Add the salt, pepper and flour and cook, stirring, for 1 minute. Stir in the beef broth and add the bay leaf to the pot. Increase heat to bring to a boil. Reduce heat to medium-low and simmer for 30 minutes.

Step 3

Taste the soup and season with more salt and pepper as needed.

Step 4

Place baguette slices on a baking sheet and toast under the broiler for 2 to 3 minutes, until lightly browned.

Step 5

Spoon soup into 4 oven-safe bowls. Place 2 baguette slices on top of each bowl of soup. Sprinkle on grated Gruyere and Parmesan.

Step 6

Place bowls on a rimmed baking sheet and transfer to the oven. Heat under the broiler for 2 to 3 minutes, until cheese is melted and lightly browned. Serve.