Slice your onions thinly. The more uniform your slices, the more evenly they will caramelize.
In a wide, heavy-bottomed skillet, heat a generous drizzle of olive oil over medium-low heat.
Add your sliced onions to the skillet and stir to coat them in the oil. Sprinkle a pinch of salt over the onions to help release their moisture.
This is where patience comes in. Lower the heat to its lowest setting and let the onions cook slowly. Stir occasionally to ensure even caramelization.
It will take about 30-45 minutes for the onions to turn a beautiful deep golden brown. This slow cooking allows the natural sugars to develop, creating that sweet, caramelized flavor.
If desired, you can add a pinch of sugar or a splash of balsamic vinegar during the last 5 minutes of cooking for extra sweetness and flavor.
Allow your caramelized onions to cool before transferring them to an airtight container.