Preheat oven to 180 degrees C. Place half the sage and all the pumpkin and garlic in a shallow baking dish. Heat half the olive oil and half the butter together until melted, pour over the pumpkin and toss to coat.
Bake for 15 minutes, or until the pumpkin is tender. Remove from the oven, remove the pumpkin (set aside) and return the garlic to the oven for another 15 minutes. Remove and set aside.
Meanwhile, heat the remaining oil and butter in a large, heavy-bottomed pan over medium heat. Add the onion and cook until soft. Add the rice and stir to coat well. Add the wine and stir until absorbed.
Start adding the hot stock, a ladleful or two at a time, and stir constantly until the liquid has been absorbed. Continue adding the stock until the rice is plump and creamy, about 25 minutes. Add salt and pepper to taste then stir in the pumpkin mixture and the cheese. Squeeze out the roasted garlic from the skin and stir it in.