I don't know about zone 6. But I planted in zone 8 in December last year and in November this year. Last year, they came up in February and did fairly well. The ones I have planted this year are up and doing fine. In my zone (8)they overwinter fine, where winter lows can dip into mid to high teens F. I am also experimenting some from seeds. They are now about 6" tall. I think, even if the winter gets to cold and harsh only the tops may perish and the roots should resume in early spring.

Some types of onions will overwinter in zone 6. I have overwintered siskiyou onions. I started them indoors and transplanted them in early September. I mulched them in October and they did just fine. Wala Wala Onions are also supposed to be a good overwintering onion but I haven't grown them. At least some types of bunching onions will also overwinter well.