Spanish Onion

Ever wondered why some recipes call for a particular kind of onion and whether another can be substituted in its place? We certainly have.
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.
The onion plant has a fan of hollow, bluish-green leaves and its bulb at the base of the plant begins to swell when a certain day-length is reached. The bulbs are composed of shortened, compressed, underground stems surrounded by fleshy modified scale (leaves) that envelop a central bud at the tip of the stem. In the autumn (or in spring, in the case of overwintering onions), the foliage dies down and the outer layers of the bulb become dry and brittle. The crop is harvested and dried and the onions are ready for use or storage. The crop is prone to attack by a number of pests and diseases, particularly the onion fly, the onion eelworm, and various fungi cause rotting. Some varieties of A. cepa, such as shallots and potato onions, produce multiple bulbs.
Types of onions
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.
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