Puree and sieve the blackberries. Put the stock and port in a stainless steel saucepan and boil and reduce for a few minutes. Add gin or brandy, fruit and boil until the sauce coats the back of a spoon.
Meanwhile, season the venison. Fry in very little butter in a hot pan for two minutes on each side.
Sharpen the sauce with a little lemon juice, taste and season. Serve the steaks sliced on a hot plate, spoon over the sauce. Serve immediately with Dauphinois potatoes or mash and a green vegetables such as kale