Spanish onions bear a close resemblance to yellow onions, except they generally are much larger. When choosing the best Spanish onions, the first thing to look for is very clean, dry skin that should be almost paper-like in its crispness. Any onions that have nicks, cuts, gouges or other marks in the surface should be avoided, because those things can cause problems inside the onion that cannot be readily seen. The root knot on the bottom of an onion should be dry, firm and free from mold, while the top of the onion should be dry and short, showing no signs of green growth. When picked up, good Spanish onions will feel heavy and firm, instead of soft or light.
One of the basic attributes to look for when choosing the best Spanish onions is the condition of the outer skin. It should be dry, golden brown in color, and mostly attached to the onion. Some onions start to shed their skin after handling, but most of the outer skin still should be present. If the onion has skin that has dark spots or dark blemishes, or if it is missing its outer skin altogether, then the onion probably should be avoided.
Even if the skin is intact, the size and handling of Spanish onions could lead to damage that goes beyond the skin. Onions that have any type of cut or gash that exposes the inner onion to the air should be avoided. Damage can be hard to see, because it is on the bottom, close to the root knot. There are instances in which an onion might seem perfectly fine except for a single gash in its surface; in reality, there is a chance the onion could be bad on the inside or that it will not last on a shelf for much longer.
In some situations, Spanish onions are sold, stored and shipped in large piles. This means some of the onions might react to the others and start to sprout despite being harvested. Onions that have sprouted, showing some green growth at the top, should be avoided, because the taste will be off. In high-moisture environments, the root knot on the bottom of the onion can develop mold that can be an indicator of rot inside the onion.
When Spanish onions are picked up and handled, they should feel very firm and heavy for their size. Light onions might have dried out on the inside, leaving dry, separated inner layers. Soft onions have started to rot and should not be purchased. One last test when choosing the best Spanish onions is the smell test, because a strong sulfur scent means the onion is old and will not taste fresh.