Heat oven to 200C/fan 180C/gas 6. Prick the potatoes several times with a fork, then microwave on High for 8-10 mins, turning once, until tender. Meanwhile, heat the oil in a non-stick pan, add the mushrooms and rosemary and cook over a fairly high heat, stirring, until the mushrooms are tender and lightly coloured. Season to taste.
Put the potatoes in the oven and roast for 15 mins until the skins start to crisp. Split open and spoon over the mushrooms. Sprinkle with Parmesan to serve.