Members of the allium family are unique in the vegetable kingdom in that they store much of their energy in short-chain sugars. Whereas most types of plants “lock up” sugars in the form of very long-chain starch molecules (which growing shoots later break down into usable sugar via special enzymes), onions store much of their sugar in the form of fructose molecules, strung together like pearls on a necklace. The nice thing about that arrangement is that it doesn’t take a whole heck of a lot to dislodge those sugars to create the sensation of sweetness. A little heat, say. Which is why cooking — especially long, slow cooking — brings out so much of the onion’s naturally sweet flavor. It also creates a rich, dark color as the sugars slowly caramelize.