The flavour of a shallot is much milder and sweeter than that of an onion, so if a recipe specifies shallots, substituting onions won't give the same results. Their lower water content means they need to be cooked more gently than onions.
How to Select
Look for firm shallots, with no soft spots, damp or moldy patches. Brown (also known as English or Dutch) shallots are the most commonly available. Small, with a light brown skin, they have a mild flavor, and sometimes have more than one bulb inside. Banana shallots are the largest variety and are named for their size. They have a smooth, tan-colored skin and are slightly milder than other types. As they're larger, they're swifter to prepare than the same volume of smaller shallots. Pink shallots have a pink skin and a crisp texture and their flavor is pungent, but not harsh.
How to Store
In a cool, dark, dry place with good air circulation- they'll keep for several weeks.
How to Prepare
Trim off the top and peel (standing the shallots in boiling water for a minute or two after trimming makes peeling easier). Then slice finely or chop. If you're using banana shallots, their longer size means you can use the same method to chop as you'd use for onions, as follows. Cut in half from top to bottom. Put the cut side down and make a number of horizontal cuts towards, but not quite reaching, the root. Then make as many vertical cuts through the shallot, again not quite reaching the root. Holding the shallot very firmly and with the knife blade at right angles to the first set of cuts that you made, slice down vertically - the shallot will fall away in small pieces as you go. Continue cutting until you reach the root, which you can now discard.