Blanch the cabbage leaves for 10 minutes in a saucepan of boiling water. Drain, plunge them immediately into cold water and then drain again.
Season the venison with salt and pepper, then place each piece on a cabbage leaf. Roll the leaf around the meat, folding in the sides as you roll. Tie the packages neatly with kitchen string or twine so they keep their shape.
Heat the oil in a large flameproof casserole (Dutch oven) over high heat, add the cabbage packages and sear for 3 minutes until coloured. Reduce the heat and cook for 5 minutes, turning the packages over occasionally. Transfer to a plate, cover with aluminium foil and leave to rest.
Deglaze the casserole with the stock (broth) by scraping the cooking juices o the bottom of the casserole with a wooden spoon or fish slice (spatula) and incorporating them into the stock. Simmer for about 5 minutes to reduce the liquid by three quarters. Add all the fruits, bring to the boil, season with salt and pepper and remove from the heat.
Remove the kitchen string and cut the cabbage packages into slices and serve with the berry sauce and mashed potatoes.