1. Heat medium saucepan over medium heat. Add butter to melt.
2. Add onions and celery and cook until soft, about 15 minutes
3. Add broth and potatoes; bring to a boil. Reduce heat, cover, and simmer until potatoes are tender when pierced with a fork, about 20 minutes.
4. Working in 2 or 3 batches, add contents of saucepan to food processor or blender. Puree until smooth. For a thicker consistency, puree until just smooth or just before smooth. For a thinner consistency, puree longer.
5. Return puree to the pan. Add milk and reheat until warm; do not boil. Season with salt and pepper.