Heat oven to 180C/160C fan/gas 4. To make the stuffing, melt the butter in a shallow pan, then fry the onion, garlic, prunes and bacon until the onion is translucent and the bacon is slightly coloured. Add the brioche and cook until combined and broken down, then add the sage, season and remove from heat.
Put a sheet of baking parchment on a sheet of foil (both should be larger than your venison). Arrange four pieces of string across the kitchen counter and put the foil and parchment on top – this will help the venison to keep its shape. Lay another four pieces of string across the parchment. Lay the pancetta on top of the paper and string, slightly overlapping the slices so that they cover an area roughly the size of your opened-out venison.
Lay the venison on top of the pancetta, skin-side down, then spoon the stuffing down the centre, patting it with your hands to make it into a sausage shape.
Fold in the sides of the venison, along with the pancetta, so they touch or slightly overlap – use the foil and baking parchment to help you. Tie the strings around the venison, making sure that the foil and parchment enclose the meat but stay outside it. Fold in the ends and tie the other pieces of string tightly around the outside of the parcel to hold it in shape.
Put the venison on a baking sheet or in a roasting tin. Roast for 40-50 mins or until a meat thermometer pushed into the joint reaches 60C, for medium rare meat. Rest for 10 mins, then remove and discard the foil and parchment. Brush the venison all over with butter and return to the oven for a final 10 mins. Leave to rest for 10-15 mins before removing the string and slicing.