The celery root is washed, peeled, and cut into small cubes. I typically use half of it if it's large, and the whole thing if it's smaller.
Then peel and chop 1 clove of garlic and 1 onion into small pieces.
Place everything in a pot and add 1/3 water, 1/3 cream, and 1/3 whole milk until it covers the vegetables.
Bring to a boil and then reduce the heat.
Let it simmer for about 20 minutes, until everything is completely tender.
The key for me to get a good purée is the following:
Drain the liquid (save it in a jug/container) and (immersion) blend the tender vegetables with a small amount of the liquid and the butter.
Add a bit of liquid at a time until you are satisfied with the consistency. It should be reasonably thick and smooth.
Season the purée with salt, pepper, and gastrique (or vinegar/sugar) until you are satisfied with the taste.
Transfer the purée to a new (and clean) pot and heat it over low temperature for a couple of minutes.