4–6 servings; adapted from Alex Lee, Daniel NYC
Bring the milk, water, and salt to boil in a large pot. Add the celery root and potato chunks, and onion, lower heat to maintain a simmer and cook until vegetables are tender, about 30 minutes. Drain and discard the liquid from the veggies.
Using an immersion blender, purée the vegetables with the butter until smooth. Alternatively, use a potato ricer or food processor.