In a 12 to 14-inch skillet, heat the olive oil over medium heat. Add the onion and squash cooking until the onions have softened and are translucent but not browned 8 to 10 minutes. Add the rice and stir with a wooden spoon until toasted and opaque, 3 to 4 minutes.
Add the wine to the toasting rice, and then add a 4 to 6-ounce ladle of stock and cook, stirring, until it is absorbed. Continue adding the stock a ladle at a time, waiting until the liquid is absorbed before adding more. Cook until the rice is tender and creamy and yet still a little al dente, about 15 minutes. Stir in the butter and cheese until well mixed. Portion risotto into 4 warmed serving plates, serving with extra cheese.