Julienne slice each onion half and separate the slices.

Heat a large pan with low sides on medium high heat. Add the oil in the pan, and when there is a wisp of smoke, carefully add the onions to the pan.

Stir periodically, for 10 minutes, allowing them to color and let the pan regain heat before stirring again

Reduce heat to medium low and continue to cook, stirring frequently for about 10 minutes.

Reduce heat to low and continue to cook until evenly caramelized, about 15 more minutes.

Turn off the heat and spread the onions out in the pan and allow to cool. They should be a rich brown and soft, but not mushy.