Preheat the oven to 350°. In a medium, nonstick ovenproof skillet, heat 3 tablespoons of the olive oil. Add the potatoes and cook over moderate heat, turning and stirring occasionally, until the slices are tender and beginning to brown, about 7 minutes. Add the onion and cook, stirring occasionally, until softened, about 3 minutes. Stir in the slices of serrano ham and season with salt and pepper.