Below is a link to an interesting article from the Washington Post that caught my eye. I was surprised that the author didn't mention shallots, also called multiplier or potato onions. They do great overwinter, and should be planted now. Normal onions, like the kind you buy in the grocery store, are called globe onions. Globe onions are planted from seed very early in spring. (To get them to grow best, you need to pay attention to day count, which I won't discuss in this post).
Multiplier onions, also called potato onions, are different because they are not planted from seeds, they are planted from bulbs. And one bulb turns into many onions, just like one little piece of seed potato turns into a whole bunch of potatoes. In the photo above, the clump of onions that you see all grew from a single bulb.
Anyway, the article in the Washington Post is interesting, and I'm all in favor of any national garden writer that encourages the use of cold frames. (I taught a cold frame-building class in my backyard last weekend -- we built 30 cold frames!)
Here's a bit from the article: