Thanks to Julie and Julia, forgotten French classics like this one are being rediscovered.
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.
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.