-Heat 3 TBS butter in a large (4-quart) saucepan over medium heat.
-Chop up the following into rough dice:
1 bunch celery (reserve leaves for garnish)
1 medium potato
-Add celery, onion, and potato; season with salt.
-Cook, stirring occasionally, until vegetables begin to soften, 8 to 10 minutes.
-Add 6 cups water or broth to saucepan; bring to a boil.
-Reduce heat to medium, and simmer until vegetables are very tender, about 20 minutes.
-Working in batches, puree soup until smooth.
-Return soup to pan; stir in 1 TBS fresh lemon juice and season with salt.
-Serve, garnished with celery leaves.