Follow on / second crops

So - my first earlies are done - will put in leeks that will take up about half the space of the spuds. Any suggestions for the other half - autumn onions to overwinter?
For my onion beds I was thinking to follow on with a few winter cabbages from the garden centre and put down some green manure for the rest of it? Any advice about the best for weed supression?

I came to the conclusion earlier today that I need another half plot to have enought space to properly do 3 bed rotation...   

Cheers
Balders