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1. Heat the olive oil in a large, heavy soup pot over medium heat and add the onion and leek. Cook, stirring, until tender, about 5 minutes. Add 1/2 teaspoon salt and the garlic and cook, stirring, until the garlic is fragrant, about 1 minute. Add the potatoes, stock or water, and bouquet garni, and bring to a simmer. Add salt to taste, cover and simmer over low heat for 45 minutes.
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2. Stir in the lettuce leaves and continue to simmer for another 15 minutes. The potatoes should be thoroughly tender and falling apart.
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3. Using an immersion blender, or in a blender or food processor fitted with the steel blade (working in batches and covering the blender lid or food processor with a kitchen towel to prevent the hot soup from splashing), blend the soup until smooth. If you want a smoother, silkier texture, strain the soup through a medium strainer, pushing it through the strainer with a pestle, spatula, or the bowl of a ladle. Return the soup to the heat, add lots of freshly ground pepper, taste and adjust salt. , Heat through and serve, garnishing each bowl with chopped fresh parsley or chives.
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Note: If serving cold, add a dollop of plain yogurt to the garnish.
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Advance preparation: The soup can be made a day ahead and reheated. You can also freeze it for several weeks. Whisk well after thawing.