In a large skillet add the 1 tablespoon of oil and squash and cook over moderately high heat, stirring occasionally, until tender, about 8 minutes. Add the sage, season with salt and pepper and cook until aromatic, about 1 minute longer. Transfer the squash to the bowl. .

In a medium saucepan, bring the stock to a simmer over moderately high heat. Reduce the heat to low and keep warm.

In a large saucepan, melt 2 tablespoons of the butter in the remaining 1 tablespoon of oil. Add the onion and cook over moderate heat, stirring occasionally, until softened. Add the rice and cook, stirring, for 2 minutes. Add the wine and cook, stirring constantly, until absorbed. Add 1/2 cup of the hot stock and cook, stirring, until absorbed. Continue adding the stock, about 1/2 cup at a time, and stirring constantly until it is nearly absorbed before adding more. The risotto is done when the rice is just tender and the liquid is creamy, about 20 minutes.

Stir the remaining 1 tablespoon of butter and the 1/4 cup of Parmesan into the risotto. Gently fold in the squash. Spoon the risotto into warmed bowls, sprinkle with Parmesan and serve.