Cut ribs from lettuce and chop. Chop leaves. Set both aside separately.
Cook leeks and lettuce stems in butter in a 5- to 6-qt. pan over medium heat with 1/4 tsp. nutmeg, the zest, salt, and pepper, stirring occasionally, until tender, about 10 minutes.
Stir in lettuce leaves and broth, cover, and bring to a boil over high heat. Reduce heat and simmer until lettuce is tender, 1 to 2 minutes.
Whirl soup in batches in a blender until very smooth, pouring as blended into a bowl. Return soup to pot and stir in half-and-half. Heat, stirring, over medium heat until steaming (don't let boil), about 2 minutes.
Ladle soup into bowls or cups and sprinkle more nutmeg on top.
Note: Nutritional analysis is per serving.