Wash and peel the celery root and cut into pieces.
Place the celery root into a saucepan, cover with cold water, add ½ glass of milk, the bay leaf, thyme and a pinch of salt. Bring to the boil, and simmer for about 20 minutes until the celery root is tender
Drain and discard the thyme and bay leaf. Bring the remaining milk to the boil. Use a food processor to blend the celery root to a smooth purée, add a small amount of the boiled milk at the same time to adjust the texture.
Season and add a small knob of cold butter. Pass the celery purée through a thin mesh strainer and keep it warm.
Notes
For a thicker texture add potato to the celery root.
To avoid oxidation of the celery root after peeling, rub it with lemon or submerge into a water bath with a lemon slice.