If you are preparing whole chestnuts from scratch, make a small slit in the skin of each one, then blanch in boiling water for about 2 minutes to ease peeling. Drain and, once cool enough to handle, peel off both the tough outer skin and the thin, brown inner skin. Now simmer in unsalted water for 15-20 minutes, until completely tender. Drain and leave to cool. Put the chestnuts (home-cooked or vac-packed) into a bowl and break up roughly with a fork, so they’re crumbled rather than puréed.
Heat the oil in a large frying pan over a medium heat. Add the onion and celery and sweat for 10–15 minutes, until softened and golden. Add the prunes, chestnuts, herbs and some salt and pepper. Mix well and cook for another 8-10 minutes, stirring occasionally. Remove the pan from the heat.
When the mixture has cooled a little, mix in all but a handful of the breadcrumbs until well combined. You can add a dash of warm water or veg stock if needed to bring it all together.
Preheat the oven to 190°C/Fan 170°C/Gas 5. Oil an ovenproof dish and pile in the stuffing, packing it down firmly. Rough up the surface a bit with a fork, then scatter over the reserved breadcrumbs and hazelnuts/pumpkin seeds if including.
Trickle over a little more oil, and bake for about 30 minutes until nicely browned and crisp on top. Serve hot.