You received a mature onion. They never form much of a bulb. They are hard to kill. If the top is green just stick it in a pot next to that window, until you can plant in the ground. They are perennials, so put it in a bed where you will not have to move it often. If you have more than one, put them about 4 inches apart each way, planting no more than the depth of the onion. Come summer, the stalk should run up and produce bulblets where the flower would normally be. Each of these will take hold wherever they touch the ground. That is why they call them walking onions.