1. Tip the rice into a large bowl, then add 500ml of the hot vegetable stock. Cover with cling film and microwave on High for 5 mins. Meanwhile, peel and cut the squash into medium chunks. Stir the rice, then add the squash and the rest of the stock. Re-cover with cling film, then microwave for another 15 mins, stirring halfway, until almost all the stock is absorbed and the rice and squash are tender.
2. Leave the risotto to sit for 2 mins, then stir in the cheese and sage. Serve topped with more grated cheese.