Onions are one of those delightful plants that you can eat at any time in its development. The thing to do with onions is plant plenty so you can have some as small green onions and also use what you need all season and have some left for storage. If you want greens, just pull the whole plant. As noted, the bulb won't get any larger if the leaves are gone.
Have you noticed that onion bulbs are layered? Each layer has its own leaf.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-