Heat oven to 180C/160C fan/gas 4. In a bowl, toss the walnuts, thyme and blue cheese together, then pack into the cavity – don’t worry if the filling sticks out of the top. Next, lay out a sheet of tin foil large enough to cover the celeriac. Put the celeriac in the middle, dot the butter on top and around it, then drizzle with honey and sprinkle with salt. Wrap the celeriac in the foil, making sure there are no gaps, and roast for 1 hr 15 mins or until soft. Once cooked, open the foil and roast for a further 15 mins until golden. Serve the celeriac on a chopping board and reserve the juices in a dish to spoon over.