You harvest onions when the tops fall over. That means they're all done growing. The tops will still be a little green when this happens, but there have been times when I haven't gotten around to harvesting them until the tops were completely dried up, and they were fine, but then again I live in a dry climate, so they probably just cured out in the ground. I've never had luck with onion sets though. If they don't work for you either, next year you may want to try onion plants instead of sets. They'll look like bunches of green onions.

Those are what I call potato onions, also called nest onions. They look like the yellow strain sold by Southern Exposure Seed Exchange. They have a growing guide that's probably posted online. Wish they were mine, but you do need to wait for dieback to harvest if you want them to store well.

Matt, not knowing where you live and what type of onion you planted makes it difficult to answer your question. There are long day onions for planting in the north and short day onions for planting in the south. If your onions are laying down, they are probably done bulbing and have started to seal off the top of the bulb. Don't water at this point. You want them to dry out. Layton and Mari--Onions do not usually flower unless it is a second season of growning for them. If they flower they are working on seed production and not bulbs. (That's why it is not a good idea to leave bulbs in the ground for next year.) Onions that bulb should be harvested and used shortly thereafter. Trying to keep those for storage will result in disappointment.

I'd harvest the onions now, and you could have been pulling and using them since they developed their round shape. Mine I harvested in September when the tops died back. You don't want to let them get frost damage since they then would not keep well. Pull them and spread them where they will be protected from temperature extremes and will get dry enough to store. When dry, cut off the tops and the roots. Depending on what type you grew, they may only stay useable for a few weeks (mild ones like Ailsa Craig or Walla Walla) or until March or so if they are specifically storage varieties. Regardless, when mine start thinking about sprouting, I make French Onion Soup, which also freezes well, to use up the remainder. Since yours are still growing, you may not have gotten a variety that is specifically bred for shorter northern season use. Brussel sprouts are supposed to be improved by frost, but you can certainly harvest them now. I am not fond of them, though so have no experience growing them.