Peel and cut the celeriac into roughly 1-inch/2.5 cm pieces. Put the milk, lemon juice, and bay leaf in a large pot, pour over 8-cups/2 liters water. Add the salt, bring to a boil, and simmer until the celeriac is very tender.
Drain, reserving 1 cup/250 ml of the liquid. Remove the bay leaf. Stir the butter and cream into the celeriac, then puree until very smooth in a blender, adding as much of the reserved cooking liquid as needed to achieve a soft, smooth consistency. Season with pepper and salt, and serve.