Chop celery finely
Chop potato into small chunks (approx 1 cm)
Chop cucumber into large chunks (approx 2 cm)
Rip lettuce into small pieces

Heat oil in pan
Add celery and potato
Stir continuously
Cook until potato starts to brown
Add cucumber
Reduce heat
Sweat in low heat for 10 mins
Add lettuce
Add 500 ml water
Bring to boil
Reduce heat
Cover and simmer for 20 mins
Smooth in blender

Lastly, reheat gently before serving.
Also, this soup can be served chilled.