Growing half an onion

I needed half an onion for some food, and didn't want to waste the other half, so I put it in a zip-top plastic bag and stored it in the fridge. And promptly forgot about it. We recently came across it again while cleaning the fridge:

The back half still has its peel:

But the roots and leaves are growing happily, unaware, it seems, that they've only half an onion.