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Heat oven to 180C/160C fan/gas 4. If the celeriac has a green sprouting top, cut it off and discard. Scrub off any dirt and trim away any frilly bits of root. Rub the celeriac all over with the oil and place on a large sheet of foil, root-side down. Scatter with the garlic, herbs and seasoning. Wrap the foil tightly and place on a tray and roast on the bottom of the oven for 2 hrs. Check after 2 hrs that the top is very tender – if not give it 30 mins more.
To serve, unwrap the celeriac and cut off the top. Loosen and mash the middle with the butter, some seasoning and squeezed out softened garlic, if you like. Serve the celeriac with a spoon sticking out of it for everyone to help themselves to.