Generally, sweet onions are in season during the late spring/early summer months. Check out this informative harvest chart for onion availability across the United States.
Choose onions with tightly closed necks that are absolutely dry, avoiding those with a thick, woody center in the neck. The skin should be bright and shiny. If you notice dark, powdery patches under the skin, pass it up as this is an indication of a common mold which will eventually spoil the flesh.
Sprouting is an indication of age and poor storage. Yet, if the sweet onions have sprouted in your pantry, you can use the green sprouts as a substitute for scallions even if the flesh may be useless.
Sweet onions have a shorter shelf life than common varieties due to a higher water and sugar content. Thus, it's important to store them properly.
Ideally, sweet onions should be stored in a cool, dark, dry location and spread out for optimum air circulation. Most growers suggest placing onions in a clean pair of pantyhose, with knots tied in between each onion, then hung in a cool, dry place. Just snip off below each knot when you need one.
Stored properly, sweet onions should last in your pantry about ten days to two weeks.
Cut, raw onion leftovers should be tightly wrapped and refrigerated to be used within a few days.
Although sweet onions are best eaten raw, they can be chopped and frozen for future cooking uses, with no blanching necessary. Frozen onions begin to lose their flavor after about twelve months in the freezer.
Cooking converts the sometimes spicy/hot all-purpose onion into sweet morsels. Cooked sweet onions are even sweeter yet, sweet enough to use in a chocolate cake.