Heat the broth and keep it warm.
Heat the oil and butter in a heavy-bottomed pot. Add onion and sauté over medium heat until translucent, about 5 minutes. Add the sage and cook for 30 seconds, then add squash and cook for 2 minutes, stirring often.
Add the wine and 1 cup of broth and bring to a boil. Reduce heat to low, cover and simmer until squash is tender, about 20 m minutes.
Stir in the rice. Add 1/2 cup of warm broth and cook, stirring frequently, until the rice absorbs the liquid. Continue adding broth in 1/2 cup increments, stirring, until rice is creamy and soft, but still al dente, about 25 minutes.
Remove from heat and stir in parmigiana. Season with salt and pepper to taste.