In a large bowl of water wash leeks and drain by lifting leeks from water into a colander. In a heavy kettle cook onion in butter over moderately low heat, stirring until softened. Add leeks and salt to taste and cook, stirring, until softened. Add curry powder and cook, stirring, 1 minute. Add remaining soup ingredients and simmer, covered, 30 minutes, or until vegetables are soft.
Cool soup slightly and in a blender or food processor purée in batches, transferring as puréed to a bowl. Season soup with salt and pepper. Soup may be made 5 days ahead and cooled, uncovered, before chilling, covered. Reheat soup, thinning with water if desired.
Make frizzled leeks:
Cut leeks lengthwise into thin strips. In a bowl of water wash leeks and drain by lifting leeks from water into a colander. Dry leeks very well between layers of paper towels.
In a saucepan at least 3 1/2 inches deep heat 1 inch oil to 375°F. on a deep-fat thermometer. Working in very small batches fry leeks (oil will bubble up quite high) until golden, about 10 to 15 seconds, transferring with a slotted spoon to paper towels to drain. Season frizzled leeks with salt. Leeks may be fried 1 day ahead and kept in an airtight container at room temperature.
Serve soup topped with frizzled leeks.
Nancy and Tim Bass - Sunburst Tomato