A spring onion is an onion harvested young, before the bulb has fully grown. That is why it tastes sweeter and gentler than a mature onion and can be used whole: the white part for the soffritto, the green part sliced raw to finish a dish.
It is one of the signs that spring has arrived at the market. In rixi e bixi, the Venetian rice and pea dish, the green tops go into the stock with the pea pods, while the white part cooks with the rice.
There are white and red ones, the latter a little stronger. Halved lengthwise and grilled, they become a very simple, sweet side dish.