Their heat disappears with cooking. This onion is the workhorse of cookery good in any heated dish or any other dish in which subtlety is not an issue. These onions are hot, but with a slightly sharper, cleaner flavor than yellows. They also tend to have a slightly shorter shelf life because they lack the pigment that affords them protection against mold and because they have proportionally more water than the yellows. They are most commonly used in Mexican cuisine.
Spanish onions are a large yellow storage onion, as round as a globe.
They usually have a slightly higher water content and so generally are less hot, sweeter, somewhat crisper and more perishable then the storage onion. They have some heat just enough to let you know that they mean business. Spanish onions can substitute for specialty sweet onions out of season. Uncooked, red onions make a handsome addition to salads of all types. However, when cooked, these onions lose some of their color.