Peel onion and chop finely. Heat butter in a pan and saute onion. Add rice and saute until rice is tranlucent.
Rinse and dry radicchio, cut into strips. Add hot broth to rice and cook, stirring frequently, until almost all liquid is absorbed. Continue to add broth in this manner and cook until rice is al dente, about 20 minutes, stirring frequently. Finely grate half of parmesan, slice the rest.
Five minutes before the end of cooking rice, add radicchio and grated parmesan cheese, mix well. Season with salt and pepper. Garnish with sliced parmesan and serve.