Haven't grown them personally but there are many discussions about overwintering onions that a search pulls up.

If
you want to try to grow the shallots for a second year I would suggest
harvesting and curing as you normally would. That would free up the
garden space so you can plant something else in that spot and it gives
you the chance to amend the area you plan to plant them later in the
year. I have only grown shallots from bulbs, not seeds. So I have no
idea how seed grown shallots will do in their second year.

Rodney

Normally bulb onions don't overwinter dependably here in Minneapolis, although once in a while they will come through, but long day varieties then tend to bolt right to flower. For hardy mostly we depend on tree onions, multipliers of some types, and the garlics. In your Zone 6 it looks like you got a shot at this though.

IIRC Johnny's like Stokes has very dependable cultural advise.

I fall seeded onions for the last two years. They are just little sprouts by the time freezing comes along. The first year they came out great. Then last year I think the fall weather was not right and they did not get enough growth before freeze. Only a handful survived and I ended up ordering onion starts in spring.