Start by checking that your chestnuts are good by dipping them in a bowl of water. Those that rise to the surface can be thrown away as they are bad.
With a knife, make a slit in the chestnuts so that they do not burst during cooking.
In a saucepan, bring the water to a boil and plunge the chestnuts in. Cook them for 3 minutes then drain them.
Preheat your oven to 200°C.
Then place the chestnuts on a baking sheet and cook for 15 to 20 minutes, depending on the size of your chestnuts. You can stir them a bit in the middle of the cooking time.
Let cool for about 10 minutes so as not to burn yourself and enjoy.