Remove the venison from the fridge and let it reach room temperature– keep any juices that may run out. Heat the oven to 250°C. Wash the potatoes and slice them into rounds.
Heat half the olive oil in a heavy pan and fry the lardons. Remove them and, in the oil and bacon fat, brown the venison all over. Remove the meat and deglaze the frying-pan with the red wine. Keep it on one side for making the sauce.
Put the venison in a medium-sized roasting tin and pour the remaining olive oil over it. Put the potatoes and chestnuts all round it and sprinkle them with olive oil. Season and roast for 15 minutes. Turn down the heat to 220°C, add the lardons and cook for a further 30 minutes, turning it after 15 minutes and sprinkling it with a little water. Remove it and the garnish to a warm dish and cover with foil.
Add the red wine in the frying-pan to the roasting pan; let it bubble, then stir in the sugar and the mustard, scraping the juices from the bottom of the tin. Season and simmer the liquid until the wine tastes mellow. Away from the heat whisk in the butter, cut into cubes, to make a smooth, silky sauce.
Serve the venison sliced, with its sauce and the roasted chestnuts, lardons and potatoes.