Rub the venison steaks with oil then season with salt.
Heat a grill or frying pan and cook the steaks over medium heat for 2-3 minutes on each side, turning them over every minute. Transfer the steaks to a plate and rest.
Thinly slice the onion and mushrooms. Add these to the pan together with some olive oil or a knob of butter. Add the rosemary sprig too. Turn down the heat and gently sauté for about 8 minutes until the onion is soft and the mushroom releases its jus.
Add the port and let it sizzle for a minute, then add the stock and reduce the sauce by half.
Put the steaks back and crumble the cheese over them. Cook for another 5 minutes or so. (Exact time depends on their thickness – they shouldn’t be overcooked).
Discard the rosemary and season with freshly ground pepper.
Serve the steaks immediately with the mushroom & cheese sauce poured over them.