Step 1

Heat butter in a large (4-quart) saucepan over medium heat. Add celery, onion, and potato; season with salt. Cook, stirring occasionally, until vegetables begin to soften, 8 to 10 minutes.

Step 2

Add 6 cups water to saucepan; bring to a boil. Reduce heat to medium, and simmer until vegetables are very tender, about 20 minutes.

Step 3

Working in batches, puree soup until smooth. (To prevent splattering, fill blender only halfway, and allow the heat to escape: Remove cap from hole in lid; cover lid with a dish towel, holding down firmly while blending.) Return soup to pan; stir in lemon juice, and season with salt. Serve, garnished with celery leaves.