Make little slits in the chestnuts then blanch or roast them to ease peeling. Simmer the chestnuts in unsalted water until completely tender (15-20 minutes).
Drain and leave to cool, then place in a basin and roughly break them up with a fork - you want them crumbled rather than puéed.
Sweat the onion and celery in the butter over a medium heat for a few minutes until softened, then add the prunes, chestnuts and parsley. Season with salt and pepper, mix well and cook for a further few minutes. Remove the pan from the heat.
When the mixture has cooled a little, mix in the breadcrumbs, liver and egg until all the ingredients are well combined. Pile the mixture into a suitably sized ovenproof dish. Bake for 30-35 minutes in a fairly hot oven (i.e. with the turkey at 180C/350F/Gas 4) until nicely browned and crispy on top