Heat the butter in a large wide pot over medium heat. Add the garlic, leeks and onion and cook, covered, stirring occasionally, until soft and not browned, about 12 minutes. Raise the heat to medium-high, pour in the wine and continue to cook, uncovered, until almost dry, about 4 minutes.
Add the potatoes, bay leaf, thyme, 7 cups water, 1 teaspoon salt and a few grinds of pepper. Bring to a boil, reduce the heat to maintain a simmer and cook, covered, until the potatoes are very tender, about 20 minutes. Remove from the heat and cool slightly.
Remove and discard the thyme and bay leaf. Working in batches, transfer the mixture to a blender and carefully puree until smooth, taking care not to fill the carafe more than half full per batch, or puree with an immersion blender in the pot. Reheat the soup over medium heat until steaming.
Whisk in the cream and lemon juice. Adjust the seasoning with additional salt and pepper. Ladle the soup into bowls. Garnish with fresh thyme leaves, a drizzle of oil and a frizzled leeks or shallot if using.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.
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