In a tall pan, add half of the butter and the chopped onion. Once the onion has become translucent, add the rice.
Toast the rice for 3-4 minutes and add all the prosecco. Stir continuously until the prosecco is absorbed by the rice and the alcohol has evaporated.
Add 1 cup of vegetable broth at the time until the rice has become al dente. Stir continuously.
Add the rest of the butter and the grated Parmigiano and stir for about 2 minutes.
Serve with shavings of Parmigiano cheese.