Preheat oven to 220 degree Celsius. Cut a large X all the way through the skin on the flat side of the chestnut. Put them on a tray and roast them in the oven for about 30 to 40 minutes, depending on size of nuts. Add a bowl of water to the oven. Shake tray several times in between to rotate chestnuts. Take them out of the oven, put them into a wet tea towel and peel them when they’re still warm.
Source: Teresa and Home Cooking