1. Cut ribs from lettuce and finely chop ribs and leaves separately. Set leaves aside.
2. In a large pot, cook lettuce ribs, leeks, celery, chopped dill, lemon zest, 1/2 tsp. salt, and 1/4 tsp. pepper with butter over medium heat, stirring occasionally, until very tender, about 15 minutes.
3. Stir in broth, cover, and bring to a boil over high heat. Add lettuce leaves, reduce heat, and simmer just until lettuce is tender, 1 minute. Set aside 1 cup of mixture.
4. Whirl soup in batches in a blender until very smooth, pouring into a bowl as blended. Return soup to pot and stir in reserved unblended soup and 1 1/2 cups fava beans. Heat, stirring, over medium heat until steaming.
5. Ladle soup into bowls. Dollop a generous spoonful of crème fraîche into each. Garnish with remaining fava beans and dill leaves and sprinkle with pepper. Add more salt and crème fraîche if you like.
How to Double-Shell Favas: Makes 2 cups 30 minutes Tear open 3 lbs. fava pods*. Add beans from pods to a pot of boiling salted water and return to a boil. Cook just until beans are tender beneath skin (slit to check), 2 minutes. Drain; rinse with cold water. Peel leathery skin by tearing beans open at rounded ends and popping out beans. *Find at well-stocked grocery stores and farmers' markets. Make ahead: All but the final peeling, up to 1 day, chilled.