1. Trim and discard root end and tough dark green tops from leek. Slice leek in half lengthwise and rinse well, flipping layers under cool running water to remove dirt. Slice thinly crosswise.
2. In a 5- to 6-quart pan over high heat, melt butter. Add leek, carrots, celery, onion, and garlic; stir often until onion is limp, 6 to 8 minutes. Add broth, Mashed Pumpkin, apple cider, apple, ginger, sage, cinnamon, and allspice; cover and bring to a boil. Reduce heat and simmer, stirring occasionally, for 10 minutes.
3. Whirl soup, a portion at a time, in a blender (holding lid down with a towel) until puréed, and pour into a bowl. Return all soup to pan and stir often over high heat until hot. Add salt and pepper to taste.
4. In a small bowl, mix 1 cup crème frâiche or sour cream and lemon peel. Ladle soup into bowls or mugs. Garnish with a dollop of crème frâiche.