I think you're supposed to cut the flower off to keep them from flowering. Then harvest much later when the green part dies. Hopefully some one will come along who knows for sure.

i had the same thing happened. When i pulled them out the onions are still small, i still have more planted so i think i will cut the flower (which is pretty ugly anyway) and let them grow some more

Thanks folks!

Just pull off the flowering part like she said and do not pick it until the entire top of the plant just falls over. Then it is ready to pick.