Cut off the ends of the celeriac and then into slices into slices about 1.5 cm. Cut the outer skin off the slices and the cut into approx 1.5 cm cubes. Keep the cubes in a bowl of cold water acidulated with some lemon juice. Repeat this with the swede and just add it to the bowl of celeriac.
Chop off the end of the pumpkin, peel and then cut into slices and as far as possible into 1.5 cm cubes like the celeriac and swede but keep in a separate bowl, and keep the rest of the pumpkin for another dish. Note that this cutting the vegetables into cubes will involve some waste.
When ready drain the swede and celeriac in a colinder, and put in an ovenproof dish. Add a couple of tablespoonfuls of olive oil, season with salt and fresh black pepper and use your hands to mix well. Put the dish into the back of a fairly hot wood burning oven.
After about 15 minutes, stir in the pumpkin cubes and turn all the ingredients well. Put back in the oven for about another 15 minutes or until cooked but not mushy turning about half way through. Keep warm until needed, and then finely chop some basil and stir in a tablespoonful or so before serving.