To roast the pumpkin: Preheat the oven to 425F. Place the cubed pumpkin in a bowl, toss with olive oil, a light sprinkle of salt, and a little chopped sage and rosemary. Spread them out in a single layer on a lined baking sheet. Roast the pumpkin for 25-30 minutes or until soft.Transfer to a blender and puree until smooth. Set aside until ready to use. Note: this can be done a day or two in advance to save time (store in the fridge).
Heat the butter and oil in a medium-sized saucepan over medium heat. Add the onion and cook until soft and translucent, about 5 minutes. Add the garlic and cook for another minute. Add the rice and cook for a minute or two to coat the grains in oil. Add the wine and cook until it has been completely absorbed by the rice. Add enough broth to just cover and rice and cook, stirring occasionally, until the broth has been absorbed. Continue adding the broth one ladleful at a time, stirring frequently, waiting to add more until after the broth has been absorbed. Repeat this process until the broth is gone (or add some extra if needed) or until the broth is absorbed and the rice is al dente, i.e. tender but has a good chewy texture. At this point stir in the pumpkin puree and just heat through, do not continue simmering. Then stir in the parmesan, butter, nutmeg, salt and pepper. Spoon into bowls and serve immediately with some shaved Parmesan for garnish.