In a large, wide saucepan, melt the butter in the olive oil over medium-high heat. Add the onions and fennel and sauté until translucent, about 4 minutes. Add the rice and stir constantly for about 2 minutes. Add the wine and cook until the wine has evaporated, 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 almost absorbed before adding the next. Stir often. Cook until the rice is tender (but not mushy) and the mixture is creamy, about 20 minutes after adding the first cup of broth.