To marinate the venison, put the red wine in a pan and boil to reduce by one third. Allow to cool down to room temperature. Place the venison in a large container and cover with the reduced wine and all the remaining marinade ingredients. Chill and allow to marinate for 24 hours.

Preheat the oven to 110°C/225°F/gas ¼. Drain the venison and the vegetables, and pat them dry. Reserve the wine. In a non-stick pan, heat the sunflower oil until smoking and sear and colour both pieces of venison on all sides to a good deep brown. This will take about 10 minutes. Transfer the venison pieces to a cast-iron casserole.

In the oil remaining in the non-stick pan, colour the drained vegetables and seasonings lightly, about 5 minutes, then transfer them to the casserole with the venison. To make the sauce, add the tomato purée to the non-stick pan and cook for 1–2 minutes. Add the reserved red wine from the marinade, and bring to the boil. Pour over the venison, vegetables and seasonings in the casserole, and barely cover with the water. Season lightly with sea salt. Bring to the boil for 1 minute, skim, then cover with a tight lid. Transfer to the oven and cook for 3½ hours.

To finish the sauce, strain the juices into a large pan and reduce on full heat to about 300ml (½ pint). Reduce the heat until the sauce barely simmers, then add the redcurrant jelly. Adjust the seasoning adding sea salt and black pepper, if necessary, and whisk in the grated chocolate. Mix the venison and vegetables back into the sauce, and reheat gently. Add the chestnuts if using.You may serve the venison from the casserole or transfer it to a nice serving dish.