In a heavy soup pot, heat the oil over medium heat. Add the onion, and cook, stirring, until it is softened, 4 to 6 minutes, and stir in the leeks. Add a generous pinch of salt and continue to cook, stirring, until the leeks are beginning to soften, about 3 minutes, careful that the vegetables do not brown. Stir in the garlic and cook until fragrant, about 30 seconds.
Add the potatoes, water, bay leaf, and about 2 teaspoons salt, then bring to a boil, reduce the heat and gently simmer, covered, until the vegetables are very soft, about 30 minutes.
Fit a food mill with the medium blade. Remove the bay leaf and purée the soup into a second pot (run the soup again through the fine blade for a finer texture, if desired). Heat the soup through, stirring to amalgamate. Taste and adjust the seasoning if desired. Add a generous amount of pepper. Whisk in the cream, taste once more, and serve, garnishing each bowl with a generous sprinkling of parsley or chives.
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