Preheat the oven to 200C/400F/Gas 6.
Lightly grease a gratin dish with the butter and add the potatoes, olive oil, fresh rosemary and crushed garlic. Season with salt and freshly ground black pepper and mix well.
Roast the potatoes in the oven for 15 minutes or until golden and cooked through.
Add the grated cheese for the final three minutes of the cooking time. Serve hot.