Heat 1 tablespoon olive oil on medium-high heat in a large skillet. Add sliced onions and cook on high heat for about 10 minutes, constantly stirring with the spatula.
The onions should start to brown, but without burning.
Reduce heat to medium and continue cooking onions for 10 more minutes, continuing to stir, as onions brown even more without burning. At this point add a pinch of salt over onions.
Continue cooking for 10 more minutes on medium or low heat, stirring occasionally to make sure onions don’t stick to the bottom of the pan or burn.
In total, you should have cooked onions for 30 minutes.
Remove the skillet from heat and sprinkle the onions with a small amount of balsamic vinegar to deglaze the pan.
Using a spatula, stir the onions, scraping the bottom of the pan and coating onions with a pinch of balsamic vinegar you just added. Remove caramelized onions to a plate.