To roast chestnuts, follow instructions in how to roast chestnuts, - in summary you score the skins, wash them, then roast on a baking sheet around 15 minutes at 425F/210C. Once cooked, allow to cool just enough to be able to handle then peel - they peel best when still warm so try to cover those you are not working on. Roughly chop the chestnuts, keeping one or two larger pieces for garnish if you like.
Dice the onion relatively small. Warm the butter in a medium pan over a medium heat then add the onion.
Cook for around 5 minutes until the onion softens, but make sure it doesn't brown (reduce heat or add a little more butter, if needed).
Add the rice and cook for a minute or two so that it becomes glossy from the butter. Add the wine and let it bubble away until it is largely absorbed.
Then add just enough stock to cover the rice and the chestnuts. Keep adding a little more stock (around ½ to 1 cup at a time) as the previous amount is absorbed, stirring now and then so that it doesn't stick. Typically, it doesn't need stirred much to start with but more towards the end.
Once the rice is cooked to just al dente and the stock has been absorbed, remove form the heat. Stir through the parmesan and parsley so that all are well distributed and the cheese melts into the risotto. Adjust seasoning to taste, if needed, and serve.