In a large, wide saucepan, melt the butter in the oil over medium-high heat. Add the onions and sauté until translucent, about 4 minutes. Add a large pinch of salt, then add the rice and stir constantly for about 2 minutes. Add 1 cup of broth and simmer, stirring until the broth is almost absorbed. Add more broth, a cup at a time, allowing each addition to be absorbed before adding the next. Stir often. Cook until the rice is tender and the mixture is creamy, 20 to 25 minutes.