Over at Serious Eats, Kenji Lopez-Alt shows how you can cut your onion caramelization time from 45 minutes to about 15 or 20 minutes.
Before we can figure out how to improve our end results, it’s important to understand exactly what’s going on when an onion browns. First, the onions begin by sweating. As they slowly heat up, moisture from their interior (they are roughly 75% water by weight) begins to evaporate, forcing its way out of the onion’s cells, and causing them to rupture in the process. This breakdown of the cells is what causes onions to soften during the initial stages of cooking.