A Cake of Parsnip and Celeriac

A Nigel Slater recipe, from his stunning book, 'Tender - vol 1'.  Not a cake made with flour, sugar, etc, but a baked dish, a gratin, almost, but with no cream or cheese, just a little stock, butter and thyme.  The smell in the flat as this baked was amazing - helped, no doubt, by the freshest, most aromatic roots from the Riverford box.  I have found an online recipe for you if you are unwise enough not to have invested in either volume of Tender.  The only change I made was to halve the amount of butter.  It made a big dish, and half of it will be reheated during the week, perhaps with some bangers.  Gorgeous.