Photograph: Martin Poole
Cook the celeriac for 20-25 minutes in boiling salted water or until tender.
Heat the extra butter and the oil in a frying pan and, when sizzling, add the sage leaves. Fry for 1-2 minutes until crisp; drain on kitchen paper.
Drain the celeriac, mash it with the 50g of butter and milk, and season. Reheat in the pan until piping hot, then add the Gorgonzola and half the sage leaves, crumbled, and stir together.
Season to taste and scatter with the remaining sage leaves to serve.