Chop the lettuce finely. In an ovenproof casserole or Dutch oven melt the butter and cook the lettuce until softened, stirring frequently. Sprinkle with flour and cook for 2 minutes, stirring. Stir in the chicken stock a little at a time. Heat gently. Season with salt and pepper. In a small bowl whisk together the egg yolks and cream. Remove the soup from the heat and quickly whisk in the yolk mixture. Serve immediately.
