I was just trying to plant some potatoes a few weeks ago. I got some green sprouting ones from the supermarket and plopped them in the ground. Nothing came up though though last year it worked (planted too late in Phoenix and couldn't enjoy the taters).

For onions, have you tried just getting some good onions from the store and letting them grow some "hair" in the dark cupboard and then planting it?
It worked for me. Best of luck getting your garden to grow!