How to Make It
Bring the stock to a simmer in a medium saucepan and keep it warm over low heat. Heat the olive oil in a large, wide casserole. Add the onion and cook over moderately low heat, stirring often, until it softens, about 8 minutes. Add the rice and cook over moderate heat, stirring, until the grains are coated with the oil. Add the wine and boil until almost evaporated. Stir in the squash.
Add about 3 cups of the hot stock to the casserole and cook, stirring constantly with a wooden spoon, until the rice has absorbed most of the stock. Continue to cook the risotto, adding the stock 1 cup at a time and stirring constantly between additions until it is absorbed. The risotto is done when it is creamy and the rice is just tender, about 30 minutes total.
Remove the casserole from the heat. Stir in the Parmigiano-Reggiano and season with salt and pepper. Spoon the risotto into 12 warmed bowls or soup plates and serve at once.