How to Make It
Bring a large saucepan of water to a boil. Add 2 tablespoons of salt and the heads of lettuce and blanch, stirring gently with a wire skimmer, for 3 minutes. Drain the lettuce and rinse under cold water. Lightly squeeze out excess water. Transfer the lettuce, still wet, to a blender and puree until smooth. Scrape into a bowl.
In a small skillet, cook the bacon over moderate heat until lightly browned, about 4 minutes.
In a large skillet, combine 2 tablespoons of the butter with the onion. Cover and cook over low heat, stirring often, until softened, about 15 minutes.
Transfer the onion to a blender. Add 1/4 cup of the chicken stock and puree. In a medium bowl, whisk the egg, then whisk in the milk and onion puree. Season with 1/2 teaspoon of salt, 1/8 teaspoon of pepper and the nutmeg. Butter four 8-ounce coffee mugs or large ramekins. Pour the onion mixture into the mugs and cover with plastic wrap.
Set a round rack in a large, wide pot. Add enough water to reach just under the rack without touching it and bring to a boil. Set the mugs on the rack. Cover and steam over low heat until the onion custards are barely set, about 22 minutes.
Meanwhile, in a medium saucepan, bring the remaining 1 cup of stock to a boil. Whisk in the lettuce puree and simmer over moderate heat for 3 minutes. Whisk in the crème fraîche and simmer for 3 minutes. Remove from the heat and gradually whisk in the remaining 3 tablespoons of butter. Season with salt and pepper.
Finely shred the reserved 2 lettuce leaves. When the custards are done, discard the plastic wrap. Scatter the bacon and shredded lettuce over the top. Serve the custards and pour in the lettuce soup at the table.