Heat a skillet over high heat until your hand feels warm when held 7.5 centimeter / 3 inches above the surface. Add the oil and saute the onions over high heat for 3 or 4 minutes, until brown. Add the sweet potatoes and cook, stirring constantly, for about 4 minutes or until the sweet potatoes are cooked through and lightly browned. Remove from the heat and season with salt and pepper and a few drops of balsamic vinegar. Serve immediately or reheat, covered, in the oven.