Heat the oven to fan 190C/conventional 210C/gas 7. Heat the butter in a small pan, add the curry powder and black mustard seeds then cook for 2-3 minutes until fragrant and bubbling. Peel and dice the celeriac and tip into a roasting tray. Pour over the curried butter, season with salt and pepper and toss together. Roast for 40-50 minutes, shaking the tray every so often, until the celeriac is golden and has taken on the colour of the curry powder.