• Set the oven at 170ºC/gas mark 4.
• Thoroughly scrub the celeriac, trim off any untidy roots and fibres, but don’t peel it. Spread a sheet of kitchen foil, large enough in which to wrap the celeriac, over the inside of a roasting tin. Place the celeriac in the middle. Brush over with the oil and run into the surface, then add the salt and rub the surface again. Add thyme sprigs, scrunch the foil together to loosely seal the celeriac. Bake for 2½ to 3 hours.

Sauce

• Coarsely grate the ginger. Finely chop the chillies, discarding the seeds if you wish. Peel and crush the garlic. Pour the oil into a medium-sized saucepan, place over a moderate heat and add the ginger, chillies and garlic. Let the aromatics cook for a couple of minutes, stirring regularly, then stir in the mustard seeds, garam masala and turmeric and continue cooking for 3 minutes. Add the coconut milk and bubble for 3 minutes.

Final bake

• Remove the celeriac from the oven, unwrap. With a sharp knife, make 2 cuts, quartering the celeriac without cutting all the way through. Pour the coconut sauce over. Return to the oven for a further 20 minutes.
• Chop the coriander and mint, then scatter them over the sauce as it comes from the oven.