I've never had this happen, but our onions have started to blossom on the top. They really aren't large enough to pull, so what's up? Do I cut off the blossom? Any advice?
Onions - blooming
If you can cut off the seed head as it first starts, you buy a little time. Once it blooms tho, just salvage what you can. Even if you catch the seedhead, think green onions. They never bulb up properly and are usually hollow.