Before blanching onions, gather some basic tools to simplify the process, particularly if you have never blanched a vegetable. Collect a cutting board, knife, a large bowl for the cold water or ice bath, a pot appropriate for the number of onions being blanched, a slotted spoon, and a lint-free cloth or paper towel for drying blanched onions. No pot lid is necessary as vegetables are generally blanched uncovered.

After bringing a pot of water to a boil, salt the water liberally. Prepare your cold water or ice bath in a large bowl and set it aside. Carefully remove the ends and skin from onion. Though it is possible to blanch onions whole, they blanch faster if you cut them into uniform quarters. Onions may also be blanched in rings, but they cook much more quickly than larger pieces. Onions sliced into rings should be as close to uniform thickness as possible for even blanching results.

Gently place the onion quarters into the boiling water with a slotted spoon. Onions only require four to five minutes at high heat to blanch to al dente -- onions cut into rings will cook in only three minutes. Since blanching creates a crisp-tender texture, avoid overcooking your onions. Onions will break down, turn mushy and fall apart if overcooked. After boiling the onions, scoop them out of the boiling water and into the cold water or ice bath, a process called shocking. When the onions have cooled completely, pat them dry with a lint-free cloth or paper towels.

Though blanching is an often overlooked way to prepare vegetables, there are many benefits to doing so. Blanching makes it easier to remove skins from fruits and vegetables, creates a crisp texture, and stops enzyme activity from turning vegetables brown. Blanching vegetables, including onions, before freezing them can slow the depletion of nutrients, and blanching vegetables makes them easier to mash or puree. When blanched foods will be cooked more later, the process of blanching is referred to as parboiling.