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1. Sauté the onions: In a large saucepan, heat the butter and olive oil over medium heat. Add the sliced onions and sauté for about 15 minutes, stirring occasionally, until softened and golden brown. Add the sugar to promote caramelization and sauté for another 5 minutes.
2. Prepare the soup: Add the broth, white wine, and thyme to the onions. Season with salt and pepper and bring to a boil. Reduce the heat and simmer the soup for 30 minutes to allow the flavors to develop.
3. Serve: Preheat the oven to 200°C. Place the baguette slices on a baking sheet and toast in the oven until golden brown. Ladle the soup into heatproof bowls, place a slice of baguette on top, and sprinkle with the grated Gruyère cheese. Place the bowls in the oven for 5-10 minutes, until the cheese is melted and lightly browned.
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