Place potatoes in a large Dutch oven with water to cover. Bring to a boil; cook 5 minutes. Drain well; slice potatoes into 3/8-inch-thick slices.
Heat oil in a large skillet over medium-high heat. Arrange half of potato and onion slices in a single layer in skillet. Sprinkle with salt and pepper. Cook, turning frequently, until onions are tender and potatoes are golden brown; drain. Transfer potatoes and onions to a serving dish. Repeat procedure with remaining potato and onion slices.