Lard the oven-ready leg of venison with olives and rub thoroughly with "Gamekeeper’s Secret". Pre-heat oven to 185°C / 365°F. Heat butter in a large skillet, place leg in skillet, and brown each side. Transfer to oven for 20 minutes, basting frequently with roast juices. Transfer meat to a platter to rest, cover with aluminium foil.
To prepare the sauce, skim fat from roast juices, add port wine and beef stock and reduce by half. Slowly whisk in the olive oil.

Heat half the beef stock and the porcini. Mix polenta with the rest of the stock at room temperature. Add polenta mixture to the boiling stock. Reduce temperature. Simmer gently at low heat, covered, for about 30 minutes. Stir frequently. Season with salt, pepper and nutmeg, then fold in the crème fraîche.