Marinate the venison overnight in 300ml of wine and the thyme.
The next day, preheat the oven to 150C/gas 2.
In a heavy-based pan, heat a third of the butter and gently fry the onions, bacon and rosemary for 10 minutes.
Tip the meat into a colander over a bowl in the sink, saving the marinade, and season well. Set the marinade aside and scatter the flour on the meat and toss lightly.
Set aside the oniony-bacon mix, leaving the bacon fat in the pan. Add the meat to the pan and fry on a medium heat, stirring regularly.
Add the carrots, prunes and garlic and fry until you can smell the garlic. Tip the onions and bacon back in and pour in the second glass of red wine and the leftover marinade.
Bring to the boil, then pour in the stock to submerge the meat. Season, add the bay leaves, then cook in the oven for 2 1/2 hours. The meat is done when it is very tender but still holding its shape.
About 45 minutes before serving, put the spuds on the boil and drain and mash when soft. Melt the rest of the butter in the milk and warm the chopped chestnut pieces in it before stirring them into the mash. Season well.
When the stew is cooked, let it sit out of the oven with the lid still on for at least 15 minutes. Skim off any excess fat and season to taste.
Serve in shallow bowls with the mash, a handful of watercress, and a little blob of cranberry jelly.