In a small saucepan over high heat, bring the stock to a simmer. Reduce the heat to low and keep the liquid hot.
In a medium, heavy saucepan over low heat, melt the butter and sauté the onion, stirring frequently, until it is soft and light brown, about 10 minutes. Add the squash and 4 teaspoons of the fresh sage or all the dried sage and sauté, stirring frequently for 2 minutes. Cover and cook until the squash is almost tender, about 6 minutes.
Uncover the pan with the squash, add the rice and stir until a white spot appears in the center of the grains, about 1 minute. Add the wine and stir until absorbed, about 2 minutes. Add 3/4 cup of the broth, adjust the heat to simmer, if needed, so that the liquid bubbles and is absorbed slowly. Stir until the liquid is absorbed. Continue cooking, adding the liquid 3/4 cup at a time and stirring almost constantly, until the rice starts to soften, about 10 minutes. Continue cooking, adding the liquid 1/2 cup at a time and stirring almost constantly, until the rice is just tender but slightly firm in the center and the mixture is creamy, about 10 minutes more.
Add the Parmesan cheese, 3 Tbs. of the nuts, the pepper and salt. Stir to mix well.
To serve, spoon the risotto into a serving bowl or onto plates. Garnish with the remaining 1 Tbs. fresh sage, if using, and the remaining 1 Tbs. nuts.