In a large heavy-bottomed pot, melt the butter and olive oil over medium heat.
Add the sliced onions, sugar, and 1 teaspoon of salt. Cook, stirring occasionally, until the onions are deep golden brown and caramelized. This will take about 30-40 minutes. Be patient and stir frequently to prevent burning.
Add Garlic and Flour:
Add the minced garlic to the caramelized onions and cook for about 1 minute until fragrant.
Sprinkle the flour over the onions and stir well to combine. Cook for another 2-3 minutes to get rid of the raw flour taste.
Deglaze and Simmer:
If using, add the white wine to deglaze the pot, scraping up any browned bits from the bottom. Let it cook for about 2 minutes.
Add the beef broth, bay leaf, and thyme. Bring the soup to a boil, then reduce the heat and let it simmer for about 30 minutes. Season with salt and pepper to taste.
Prepare the Bread:
While the soup is simmering, preheat your oven to 350°F (175°C).
Place the baguette slices on a baking sheet and bake them for about 10 minutes until they are dry and lightly browned.
Remove the baking sheet from the oven and set aside.
Assemble and Serve:
Preheat your broiler.
Ladle the hot soup into oven-safe bowls. Place a slice or two of the toasted baguette on top of each bowl of soup.
Sprinkle a generous amount of grated cheese over the bread and soup.
Place the bowls on a baking sheet and broil in the oven until the cheese is melted, bubbly, and golden brown, about 3-5 minutes.
Carefully remove the baking sheet from the oven. Let the soup cool slightly before serving.
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