Spring Onions are the spring (or sometimes fall) versions of the cured bulbs you're used to seeing year-round at the grocery store. If left to grow further, the onion greens would die off in the heat of the summer, and then after the bulbs had thickened up a bit, they'd be hung up to dry, and the familiar papery cellulose layers would form a protective layer that enables the onion to stay in storage for a few months up to a year, depending upon the variety. At this stage in the plant's life, the whole plant is edible and can be chopped up much like a scallion. It has a sweeter, milder flavor than when the bulbs are fully cured.