Peel the celeriac, chop into 2.5cm/1-inch chunks and place in a pan of cold water. Place on the hob, bring to the boil and simmer until tender, about 20 minutes. Mash the celeriac with 100g/3½oz of the butter and season with salt and freshly ground black pepper. Keep covered and warm.

Meanwhile, peel, halve and thinly slice the onion lengthways into semicircles. Heat the olive oil in a heavy-bottomed frying pan and cook the onions on a medium high heat until soft and lightly coloured.