Chop thinly the onion. Put the oil into a pan -not too high- and let it cook without boiling. Brown the onion in oil for 1 minute and then stir in the rice and cook continuously turning. After 1 minute pour the hot broth gradually, to make it absorb slowly. After about 10 minutes taste the rice, and if it is “al dente”, add salt. Then add the butter and white truffle and let it cook over a high heat for at maximum 1-2 minutes. Serve the risotto in hot dishes and then sprinkle with slices of white truffle.