Heat the oven to 200C/180C fan/gas 6. Put each chestnut flat-side down on a chopping board. Hold the chestnut carefully and, using a sharp knife, cut a long slit or a cross in the top. Make sure you cut through the shell but not the nut inside. The shell is tough and flexible like a skin rather than rock hard like other nut shells, so a small kitchen knife works well.
Tip the nuts onto a baking sheet or into a roasting tin and arrange them cut-side up. Roast for 30 mins. The cuts should open up and the shell will start to peel back.
Leave the nuts in the tin to cool down to warm – they will be very hot inside. Put a board on top of the tin while they cool to trap the steam and make them easier to peel. Serve as they are for people to peel, or peel them yourself to use in a recipe.
If you peel them, you may want to remove the inner, slightly fluffy, membrane as well. If the membrane is difficult to get off (it shouldn’t be if you steam them as they cool), you can soak the nuts in boiling water for a minute to loosen. Drain and peel.