Should I let them keep growing? Will they be usable? I have lots of both.
thanks for any advice
Onions are biennials and will flower and set seed if allowed to overwinter. Potatoes will regrow, assuming they make it through the winter.
In my opinion it would be best to harvest them when mature and store them rather than allow them to overwinter.
But I'm not entirely sure if you are thinking about allowing them to overwinter or if you are asking about plants that have overwintered and are growing.
As mentioned by forgotten, the potatoes will regrow, which means that they will produce again. You can harvest the new crop in a couple months -- so you'll be eating new potatoes, not the old ones that overwintered.
The onions will go to seed, and the old bulb will rot. Are they usable now? Then I'd say eat them. Leave a few in to let them go to seed and self seed, and you'll get lots of volunteers.
Sorry, I should have explained better.
These are plants that overwintered this past year--I didn't know they'd be there but they are up and growing well. Glad to hear I can just harvest the potatoes like usual. The onions I don't think have much of a bulb so I will just let them re-seed.