If you are using frozen steaks then take out of the freezer the day before and allow them to defrost in the fridge overnight. Using kitchen towel, pat the steaks as dry as possible and season on both sides with a little salt and freshly ground pepper.
Heat the butter and sunflower oil in a heavy frying pan until the butter has melted and started foaming.
Reduce heat to medium high and use tongs to place the steaks on the pan. Cook for 3-4 minutes and turn over once nicely browned on one side (lift the edge with the tongs to check).
Cook for a further 3-4 minutes on the other side for medium cut steaks – you will need to cook longer if the steaks are really thick.
Once the steaks are nicely browned on both sides transfer them onto a plate and cover with foil. Leave them to rest for 5-10 minutes. Don't skip this 'resting' stage! Set aside while you are making the sauce.
Deglaze the hot pan with a good splash of Port. Add the jelly and stir to dissolve. Add the berries and briefly cook until they start to soften and release their juices.
Sprinkle with the chopped rosemary and serve the steaks with the sauce on the side – mashed potatoes (or celeriac) and steamed spinach make great sides or stick to a green salad a lighter meal.