In a saucepan, bring the stock to a simmer. Heat the remaining 1 tablespoon of oil in the skillet until shimmering. Add the onion and cook over moderate heat, stirring frequently, until softened, about 7 minutes. Add the rice and cook, stirring until the rice is coated with olive oil, about 2 minutes. Add 1 cup of the hot stock and cook, stirring frequently, until the rice has absorbed most of the stock, about 3 minutes. Continue to cook the risotto, adding 1 cup of stock at a time and stirring constantly between additions until the stock is absorbed. The risotto is done when it is creamy and the rice grains are just tender, about 20 minutes.