Place the butter in a large saucepan and melt over medium heat. Add the sage, rosemary, and garlic, and cook until fragrant, about 1 minute.
Add the riced cauliflower and mashed butternut squash and cook for 5 minutes or until the cauliflower begins to soften.
Add the sherry and cook for another 7 minutes, or until most of the liquid has been absorbed and the cauliflower is soft.
Stir in the Parmesan, Mascarpone, and grated nutmeg. Continue to cook over medium heat, stirring occasionally, until the cheese is melted and the risotto is creamy, about 5 minutes.
Season with salt and pepper, to taste. Garnish with additional grated cheese and fresh basil (optional).
