Combine squash, leeks, apples, broth and water in a 4-quart slow cooker.
Cover and cook on high setting for 4 hours, or until squash and leeks are tender. Carefully purée the hot soup, in 3 or 4 batches, in a food processor or blender until smooth. Add seasoned salt and white pepper. Garnish with nutmeg and sour cream.
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