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Yes you can grow onions etc in containers,it depends on what sort of garden you want..if like me you don't mind old plastic baths (available free from skips etc everywhere) or similar recepticles making your property look like Steptoe's yard then fine (fill with soil from somewhere else plus spent mush comp\grow bags etc.)If however you are going for entry in the National Open Garden Scheme,you might have to get busy with the hypertuffa.Anyhow I've grown allsorts like this,it saves bending down too much,and reminds me of when I was a kid.For obvious reasons,specially good for carrots.

I've grown both onions and garlic in containers - only problem I found was them sometimes getting a bit waterlogged so would just make sure the drainage is good. Ones done in pots were a bit small but a bath sounds ideal! :)

I've only grown garlic in pots.

They've always done well. Some of them even grew legs and walked away... (quite literally, I've never found the pot or the 6 garlics that were in it!)

If they're in pots near the back door, your can cut off some of their greenery and use it in cooking. Garlic scapes are absolutely delicious!