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.

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.

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.