Preheat oven to 450.
Cut the squash in half, scoop out the seeds and strings, and put them in saucepan. Brush the squash flesh with butter, sprinkle with a bit of salt, and place on the cookie sheet, flesh side up. Bake about 45 minutes, until tender. Set aside to cool, then spoon the flesh into bowl, discard skins, and mash squash until smooth.
Meanwhile, add stock and ½ t salt to the seeds and strings and heat over high heat until it boils. Cover, lower heat, and simmer for 20 minutes.
Set strainer over bowl and pour the liquid through. Use spoon to squish the strings and get all the liquid out. Measure the liquid back into the saucepan and add water until the total liquid equals 5 cups. Heat to a simmer, cover, and turn off heat.
In the heavy-bottomed saucepan, heat 1 T butter with olive oil until butter melts and is no longer foamy. Add onion and sauté until soft, stirring occasionally, about 4 minutes. Add garlic and sauté another minute.
Stirring constantly: Mix in rice and cook for 1 minute. Add wine and cook about 2 minutes, until absorbed. Add 1 c of broth and cook until absorbed, then add another c, and repeat until rice is soft and creamy with just a bit of hardness right at the center. Add more hot water if needed to get the texture just right. This should take about 25 minutes total.
Remove from heat and stir in mashed squash, Parmesan, sage, and the last T butter. Mix until butter is melted, then add nutmeg and additional salt, if desired. Serve immediately, with Parmesan cheese for sprinkling.