Caramelizing onions takes nothing but a heavy-duty pan, a passel of onions and some patience. Here's my method for getting onions completely caramelized to have on hand for burgers, pizza, dips and more.
Heat the oil in a large skillet, at least 12 inches wide, over medium-high heat. Add the onions and stir with a wooden spoon to coat with oil. Allow to cook, stirring to prevent the onions from browning, until they are soft and translucent, about 10 minutes. Season with salt and black pepper, if desired.
Reduce the heat to medium. Continue to cook until the onions grow limp, stirring at intervals of about 5 minutes, until they onions have completely softened and begin to brown, about 15 minutes more.
Reduce the heat to low. This is the critical period of cooking when the sugars in the onions will begin to brown. Leave the onions undisturbed as long as possible to encourage browning. Stir at intervals of about 10 minutes and watch them during this last 40 or so minutes of cooking to prevent scorching.
Continue cooking the onions over low heat, stirring and scraping the bottom of the pan as necessary. The onions are fully caramelized when they are a very silken mass and deeply caramel in color. Remove from the heat and store until ready to use for up to 5 days.
You can also caramelize the onions in 1 tablespoon butter with 1 tablespoon oil for flavor.
For a variation, add fresh or dried herbs, such as rosemary, thyme or lavender to the onions during the last 10 minutes of cooking.