In a heavy saucepan, heat the butter and olive oil and saute the venison over a medium heat in batches until brown. Set aside. In the same saucepan, fry the bacon and then add the onions and cook until soft, taking care not the burn them. Add the celery and mushrooms, stir, and then return the venison to the pan. Add the beer, season well and cook for two hours with the lid on. Add the prunes and cook for another half hour, until tender. Serve this with the roasted parsnip and carrots, or some celeriac mash or puree.