Preheat the oven to gas 8, 230°C, fan 210°C. Put a large, lidded ovenproof dish (such as a cast-iron casserole) into the oven and allow to heat up for 30 mins. Remove the dish from the oven and remove the lid, taking care as the dish will be very hot. Lift up the baking paper under the dough and carefully drop into the hot dish so that the paper folds up the sides. Use a sharp knife or scissors to cut a cross in the top of the dough.