1. Pour oil into a 5- to 6-quart pan over medium-high heat; when hot, add onion and garlic and stir often until onion is limp, 5 to 6 minutes. Add rice and stir until opaque, about 3 minutes.
2. Add wine and stir until it has been absorbed, 1 to 2 minutes. Add radicchio and 3 1/2 cups broth and bring to a boil over high heat, stirring often. Reduce heat and simmer, stirring often, until rice is tender to bite, 15 to 20 minutes. If a creamier consistency is desired, stir in about 1/2 cup more broth. Stir in cheese; add salt and pepper to taste. Serve immediately.