In a 4 quart sauce pan, cook the onion in the olive oil until slightly browned and soft.
Add the potato, veggie stock,lettuce and salt to the pan and bring to a boil. The lettuce will fill the pan to overflowing at first, but it will cook down - be patient. After the soup comes to a boil, turn it down and allow to simmer for about 20 minutes - or until the potatoes are soft.
Remove from heat. Squeeze the lemon juice into the soup. Put the tofu into the soup and mix completely with an immersion blender.