Melt half the butter in a large saucepan over a low heat. Add the onion and cook for 10 minutes until softened. Add the chilli and cook, stirring, for 1 minute. Add the pumpkin and cook, stirring constantly, for 5 minutes.
Add the rice and cook, stirring, for 2 minutes. Add a large ladleful of the simmering stock. Slowly simmer, stirring constantly, until the stock has been absorbed and the rice parts when a wooden spoon is run through it. Add another ladleful of stock and continue to cook, stirring and adding the stock in ladlefuls, for about 20 minutes until the rice is creamy and almost tender to the bite and the pumpkin is beginning to disintegrate. Season well with salt and pepper.
Remove from the heat and add the chopped sage, remaining butter and the Parmesan. Stir vigorously for 15 seconds. Cover with a tight-fitting lid and leave to stand for a few minutes. Serve garnished with sage sprigs.