You can buy pearl onions in several forms: raw and in a bag, peeled and frozen, or peeled and pickled in a jar. All have their uses. Purchasing them raw and unpeeled gives you the most freedom, but you have to peel them, and peeling a tiny onion is not at all like peeling a big onion.
Since there is only so much onion in a pearl onion, most people wish to keep them whole, both for roasting and pickling. This means the usual “slice in half and chop off the stem, then peel the skin off,” way of doing things is not going to work.
In fact, the first thing you’ll do is slice off the root, which is usually a bad strategy for peeling onions.
To successfully peel a pearl onion, you need to think of it like a tomato, and by that I mean you need to blanch it. Just lop off the tiny stems of every onion you wish to peel, plop the onions in boiling water for 30 seconds, then immediately plunge them into an ice bath.
Let them cool for about a minute, then squeeze the stem end of the onion to reveal a perfectly peeled pearl onion ready for roasting, stewing, or pickling in aquavit. (Save the tiny peels for stock. Even though they are small, they still have a mighty amount of flavour.)