Also known as baby onions or button onions, pearl onions are a variety of tree onion, often used for pickling as cocktail onions. Pearl onions are also used in recipes like succotash and onion relish because of their sweet, mild flavor and their attractive, diminutive size.f you are following a recipe and it simply says onions instead of a specific type, use yellow onions. Yellow onions are frequently used to make French onion soup because they caramelize well and have a rich, strong, and delicious flavor.