Preheat the oven to 180°C, gas mark 4. Put the potatoes in a bowl and add the garlic, chopped sage, olive oil and garlic salt. Toss to coat.
Layer in a roasting tray and pour over 110ml water. Cover with foil and bake the potatoes for 20 minutes. Increase the temperature to 200°C, gas mark 6. Remove the foil and scatter with the cheese and sage leaves. Cook until the cheese has melted.
Meanwhile, set a large frying pan over a high heat. Rub the steaks with the sunflower oil. Fry for 3-5 minutes on each side, or until done to your liking. Baste with the butter for the last minute. Rest.
Slice the steaks and serve with the baked potatoes and lamb’s lettuce.