Can I save and use the seed from onions that have bolted?

I tried this with an onion that had sprouted leaves in my cupboard, and my onion ended up sprouting two giant flower stalks and going to seed with no trouble. Do make sure you give it enough space - the roots don't go deep, but the whole thing gets larger than you'd expect. Get a pot with two or three inches of extra space around the onion. Leave the onion sitting up in shallow water to encourage root growth for a week or so first (I used a wide glass so the sides of the glass prevented it from falling over). You can just sit the onion on top of the soil with the bottom a bit buried.