All these onions vary slightly in flavor, texture, and color, but can usually be substituted for one another. In terms of cooking, they will all behave the same in the pan.
When buying onions, go for ones that feel heavy in your hand and firm. Avoid soft onions or ones that have a sharp oniony odor before peeling. These are indications that the onion is old. Except for sweet onions, all these onions can be stored for several weeks in a cool, dark pantry or cupboard.
Yellow Onions - We consider this the all-purpose onion, and personally, it's the one we use most often. Yellow onions have a nice balance of astringency and sweet in their flavor, becoming sweeter the longer they cook. They are usually fist-sized with fairly a fairly tough outer skin and meaty layers. Spanish onions are a particular kind of yellow onion and we find them to be slightly sweeter and more delicate in flavor.
White Onions - These onions tend to have a sharper and more pungent flavor than yellow onions. They also tend to be more tender and have a thinner, more papery skin. They can be cooked just like yellow onions, but we also like them minced and added to raw salsas and chutneys.
Sweet Onions - Walla Walla and Vidalia are the most common kinds of sweet onions. These onions lack the sharp, astringent taste of other onions and really do taste sweet. They are fantastic thinly sliced and served in salads or on top of sandwiches. They can range in color from white to yellow and often have a flattened or squashed appearance. Sweet onions tend to be more perishable and should be store in the refrigerator.
Red Onions - With their deep purple outer skin and reddish flesh, these are really the odd-guys out in the onion family. They are fairly similar to yellow onions in flavor, though their layers are slightly less tender and meaty. Red onions are most often used in salads, salsas, and other raw preparations for their color and relatively mild flavor. The lovely red color becomes washed out during cooking. If you find their flavor to astringent for eating raw, try soaking them in water before serving.
Do you have a favorite kind of onion?
(Originally published October 21, 2010)