In a 3-quart pot, cook the bacon over medium heat until lightly browned. Add the onions and cook, stirring, until wilted. Add the butter and stir until melted. Add the lettuce and saute for 2 minutes. Sprinkle the flour into the pot, then stir until well blended.
Remove pot from the heat and add hot chicken broth, tomatoes, nutmeg and pepper. Heat the soup to boiling, stirring constantly, then reduce the heat and simmer gently for 5 minutes. Add the cream and mix well.
Mix together the mayonnaise and sour cream. Serve each portion of soup with a dollop of mayonnaise mixture topped with crumbled bacon.