Heat 1/2 cup olive oil in a large skillet over medium-low heat. Add 1/2 of the potato slices, and cook, stirring occasionally, until just tender, 15 to 20 minutes. Transfer potatoes to a large bowl, leaving oil in the skillet. Add remaining potatoes to the skillet; cook, stirring occasionally, until just tender, 15 to 20 minutes. Transfer potatoes to the bowl, reserving oil in the skillet. Season potatoes with salt and pepper.

Meanwhile, heat 2 tablespoons olive oil in a separate skillet over medium heat. Stir in onion rings, and gently cook until soft and golden brown, about 15 minutes. Spoon onions onto a plate, and allow to cool.

Whisk eggs in a large bowl until smooth. Stir in cooled onions, roasted peppers, and serrano ham. Gently fold in cooked potatoes.

Heat the large skillet with reserved oil over low heat. Pour in egg mixture, and gently cook until the sides have started to set and the bottom has turned golden brown, 8 to 10 minutes. Loosen tortilla with a spatula if needed, then carefully slide onto a large plate. Turn the skillet upside down and place onto the uncooked side of tortilla. Turn the skillet right-side-up, and remove the plate. Return the skillet to the stove, and continue cooking until tortilla has set in the center, about 4 minutes.

Slide the tortilla onto a serving plate and allow to cool to room temperature. Cut into six wedges and sprinkle with parsley to serve.