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.

thank you for the information! I think it threw me off that the tops never died back on the onions. They are a storage onion, so that should be helpful. I will pull them today!