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.