Preheat the oven to 190°C.
Blend the onion, garlic & celery in a small food processor until fine. Heat the butter in a pan and fry the onion & celery mixture until soft. Add the sherry then take off the heat.
Blend the bread and herbs roughly, then add the chestnuts and process for just a few seconds - the mixture should remain quite coarse!
Mix the bread & onion mixture then add the cranberries. Season with salt and pepper, then combine it into a dough.
Form about 12 balls. Put these in a baking tray and bake for 20-25 minutes until golden.