Heat the olive oil in a pan, add the onion and sauté for approx. 1 min. Add the rice, sauté briefly, pour in the prosecco and reduce completely. Stir in the stock gradually so that the rice is always just covered with liquid, season, allow to cool slightly.
Mix the egg and parmesan into the lukewarm rice, leave to cool. Place 1 heaped tablespoon of rice in the palm of your hand, create a hollow, place 1 parmesan cube in the hollow, cover with a tablespoon of rice and shape into a ball. Repeat the process with the remaining mixture. Place the egg, flour and breadcrumbs in three shallow dishes. Toss the rice balls in the flour, then dip in the egg and finally roll in the breadcrumbs, pressing firmly on the crumb coating. Fill the pan approx. 3 cm deep with oil, heat up. Shallow fry the rice balls in batches over a medium heat for approx. 4 mins. all over. Remove and drain on paper towels.