Collecting Seeds

I have let some of last years onions and Leeks go to seed. Although the flowers are now finished and closed I have opened one of them up and contents are still green - so when do I collect the seeds and how do I do it

 

Andrew, wait until the seed head is starting to go brown and dry out, then you can either tie a paper bag over the seed head to catch the seed, or what I would do is cut the head of with a long stem tie a paper bag over it and hang it up in a dry shed.
Occasionally leeks produce plantlets on the seed head.

 

Thanks Pete2255