Puree the tomatoes with their juices in a food processor and set aside.
Put the bacon in a heavy medium sized pot and cook until crispy. Scoop out with a slotted spoon and set aside, leaving the drippings in the pot.
Add the chopped onion to the pot and cook for about 5 minutes or until onions are tender, stirring every now and then.
Add the pureed tomatoes and chicken broth and bring to a boil, then turn the heat down and simmer for 30 minutes.
Carefully transfer the soup to a blender and puree until smooth (or even better use an immersion blender and puree right in the pot!).
Return the soup to the pot, taste and add salt and pepper if it needs it.
Ladle the soup into bowls, garnish with the cooked bacon and chopped lettuce and serve!