Bring stock, water, celeriac, potatoes, apple, shallot, bay leaf and bring to a boil in a large pot.
Cook until tender, about 10 minutes.
Strain, reserving liquid but discarding bay leaf.
Return celeriac, potatoes, apple, and shallot to pot, and add 3/4 cup reserved cooking liquid.
Coarsely mash with a potato masher.