Steam the potatoes for 20 minutes, or until you can easily pierce them with a paring knife. While potatoes are steaming, heat the olive oil in a 10-inch skillet over medium heat. Add the red onion, and saute until onions are softened but not browned, about 10 minutes. Turn off the heat under the skillet and add the vinegar, capers, mustard, pepper and salt. Set aside.