Peel the onion and chop finely. Discard the outer leaves of the radicchio, cut the radicchio into quarters, rinse and cut into fine strips. Put aside some radicchio strips for garnish.
Heat 1 tablespoon butter in a wide saucepan and saute the onion and radicchio in it briefly. Add the arborio rice, pour in the white wine and let evaporate. Gradually pour in the beef broth and simmer for 20 minutes, stirring occasionally. The rice should be cooked but still al dente. Stir in the remaining butter and the Parmesan into the risotto, and season with salt and pepper.
Arrange the risotto on warmed plates. Garnish with radicchio and some Parmesan slices.