Heat the olive oil in a skillet over medium heat. Add the onions and sauté until they begin to turn golden and soften. Add a few tablespoons of water and cook until evaporated. Add 1 tablespoon of vinegar and stir to avoid sticking. Add the remaining water and cook until evaporated. Add the remaining vinegar, 1/2 teaspoon salt and 1/2 teaspoon pepper.