The essence of an authentic onion soup au gratin is, well, the onions—or more specifically, caramelized onions, which are the key ingredient in this classic French country-style soup. Another key “ingredient” is a generous amount of patience.
When onions caramelize, they transform from a crisp, bulbous root into a slippery, squidgy tangle of mahogany ribbons. To achieve this desired sludge, a good amount of time is required, and this is where you must rely on patience. You may find recipes for caramelizing onions in which the onions are fried on the cooktop and browned in a mere half-hour. While they may appear to be caramelized, they are, in fact, not.
