Soak the dried porcini in the boiling water in a small bowl for 15 minutes. Meanwhile, heat one-third of the butter and the oil in a flameproof casserole and add the venison, a few pieces at a time. Add the onion and fry over a high heat for 5 minutes, stirring until browned.
Stir in the flour then mix in the wine, stock, mushrooms and soaking liquid, tomato purée and redcurrant jelly. Season then bring to the boil and cover the dish. Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 1¼ hours.
To make the topping, cook the potatoes in boiling water for 15 minutes until tender. Drain and mash with half the remaining butter and the milk. Stir in the horseradish, to taste, the chives and a little seasoning. Spoon the venison mixture into a 1.5 litre (2½ pint) pie dish. Dot the potato over the top then cover and chill until required.
When ready to serve, remove the cover from the pie and dot the potato with the remaining butter. Stand the dish on a baking sheet then cook in a preheated oven, 190°C (375°F), Gas Mark 5, for 45–50 minutes until golden brown and piping hot. Serve with green beans.