Heat a pan over medium-high heat and add the olive oil. Then add the sliced onions and sauté for 2-3 minutes, stirring until they start to turn translucent.
Add the balsamic vinegar. Turn the heat down to medium-low and sauté, stirring occasionally, for another 5 minutes or until the onions are transparent and a little browned on the edges. Season with salt and pepper to taste.