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To be honest, pearl onions are available pretty much year-round at the same quality level, but they're especially useful in winter when other vegetables aren't in their prime. Available in white, yellow, or red (just like their full-sized brethren!), they are generally milder than full-sized onions and take on a noticeable sweetness when cooked.
My favorite way to prepare them is to slowly cook them down in butter until they're deeply browned and sweet. They also go great in stews and braises, like a good coq au vin. The only annoying part is that it takes an awful lot of work to clean something that gets eaten in a single bite.
Here's a little trick to help you remove the skin easily with your fingers by blanching the onions first. Watch the video for full instructions.
P.S. Peeling pearl onions isn't too hard, especially with this trick, but here's a little secret: frozen peeled pearl onions work really well for almost all applications. I often buy them when feeling a bit lazy. shh.
Shopping and Storage
Most people don't have too many options when looking for pearl onions. They come in plastic mesh bags already bunched together. If you have the option of picking them loose, look for onions that are identical in size to help them cook evenly. The onions should also be firm with no soft spots.
Unlike regular onions, where the variation in flavor between colors is quite pronounced, with pearl onions, the difference between yellow, white, and red varieties is not too significant.