Gently clean the truffle with a soft brush, then a day or two beforehand put it in a bowl with the dry rice and cover tightly for the rice to take on the perfume.
Carefully sweat the finely chopped onion in oil and butter for 5 minutes until transparent.
Stir in the rice and stir until the grains are glistening.
Now add the glass of Noilly Prat (or Vermouth) and stir until the rice has absorbed the liquid.
Now add a generous ladle of hot stock, and stir with a wooden spoon until it too has been absorbed.
Add a second ladle and stir again gently until absorbed.
Repeat until the stock has been used and the rice is creamy but not dry (about 20 minutes).
Remove from the heat, stir in the butter and season with salt and pepper. Pile into individual deep plates and top with Parmesan and truffle slivers.