Preheat the oven to 220 degrees/gas 7.

In a large bowl, mix the sliced celeriac with the cream, thyme and 250ml of the milk. Mix the remaining 50ml of milk with the cornflour, add it to the celeriac and mix well. To assemble, grease a large gratin dish and place layers of the sliced celeriac into it, building it up layer by layer just as you would a potato dauphinoise, dotting the chopped prunes in and around the layered celeriac as you go. Dot the top with generous knobs of butter and bake in the oven, covered in tin foil, for one hour. Remove the tin foil, add a few more knobs of butter and continue baking till golden on top – about another 15 minutes or so; the celeriac should be tender. Serve with the beef and radicchio.