Hi Chao.
If they're what my grandparents called multiplying or bunching onions we had them all year long. They planted them close together and would thin them as they were needed for eating or cooking.
When the bunches got large they would divide them..give some away, replant some and dry the rest and store in a cool dry place.
You might want to try leaving or replanting a couple of the largest bulbs and see if you can grow them all year also.
I've been looking for some...that's how I found your post...lolo
Sandy