Cut the meat into strips about the size of your thumb if possible. Some of the trimmings may, of course, be slightly ragged, triangular or cubed but this is the size and shape to aim for. Dust with coarsely ground black pepper and 3 to 4 crushed juniper berries, moisten with the cider, cover and leave to marinate for 24 hours. Crush a garlic clove, mix it with the soured cream or yoghurt and leave it to infuse in a cool place for 24 hours. Carefully drain and dry the venison, reserving the marinade. Slice the mushrooms thickly and saute them in a little very hot butter. Remove and keep hot. Then saute the venison briefly, searing it nicely but keeping it succulent and pink within 2 minutes is plenty. Then let the venison rest in a low oven where it will go on cooking a little without toughening. Start making the sauce straight away or wait for several minutes if you want the meat to lose its pinkness. To make the sauce, let the marinade liquid and the stock bubble away in the frying pan until reduced to just 2 or 3 spoonfuls. Blend in any juices that the venison has exuded and bubble again briefly. Then beat in the garlic-flavoured soured cream, away from the heat. Return the pan to a low flame to warm the sauce. Season well, scatter with chopped chives and serve on very hot plates.