Heat the oil in a frying pan, cook the venison for 5 mins, then turn over and cook for 3-5 mins more, depending on how rare you like it and the thickness of the meat (cook for 5-6 mins on each side for well done). Lift the meat from the pan and set aside to rest.
Add the balsamic vinegar to the pan, then pour in the stock, redcurrant jelly and garlic. Stir over quite a high heat to blend everything together, then add the blackberries and carry on cooking until they soften. Serve with the venison, celeriac mash (see below) and broccoli.