Is a pearl onion different from a petite white onion?
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I'm not sure. . .a pearl onion is the size of a garbanzo bean or a little bigger, up to 1/2" diameter. I've never seen a petite white onion, but if it's bigger than the tip of your little finger, you could either halve (or quarter) it or allow it to cook for a few minutes longer.
A pearl onion is smaller than a petite white onion. They can be used interchangeably but realize the size difference will be a factor to some degree.