Answer: Generally speaking, onions grow tops in the cool weather and form bulbs when the weather warms. If you harvest too soon, the bulbs will be small. Once the tops begin to turn yellow, bend them down to stop the flow of sap. When the tops turn brown, pull or dig the bulbs on a sunny day and leave them to dry in the sun. When the outer skins are thoroughly dry, wipe off any soil and remove the tops (unless you plan to braid them), then store in a cool, dry, airy place. You were right in cutting off the flowering stem - allowing it to grow would have diverted energy from the bulb. If you're really curious, and can't wait for the tops to yellow, you can sacrifice one plant by digging it up to see how large the bulb has grown. Otherwise, just leave them alone until the tops start to die.
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