In a large 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 sliced lettuce and sauté for 2 minutes. Sprinkle the flour into the pot then stir until well blended.
Remove the pot from the heat and add the 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 the sour cream. Serve each portion of soup with a dollop of this mixture topped with crumbled bacon. Enjoy