With your wet clay, go for wild asparagus, promised to grow anywhere, thinner spears, but very good flavour(Seeds of Italy).
I'm quite unsure about onion being any good candidate there, spring is too early for leeks out, sow them in a tray, but chives and welsh onions should be OK. You can so beetroot and lettuce in April, happily, also a lot of other salads.
If you keen on new pots, rather try them in a bin bag of compost or the bag of compost itself.