Firstly peel the celeriac and use a knife to cut out any dirty skin from the holes on the surface. Rinse off the peeled celeriac.
Slice into cubes.
Add the celeriac, onions (or shallots), and garlic to a large pot. Pour in just enough vegetable broth to cover the veggies. Bring to a boil then cover, reduce heat, and cook for 25 minutes.
Drain off most of the broth. Add the milk and butter to the pot and heat for a few minutes until melted. The goal here is to make sure you’re not adding cold milk or butter to a dish that’s supposed to be served warm.
Carefully transfer to a blender.
Blend until smooth. Taste and adjust seasonings as desired.