Method:
Heat 3 tablespoons oil over medium heat in an 8-inch skillet. Add potatoes, salt and pepper and cook until tender, about 15 minutes. Remove potatoes from skillet, drain on paper towels, and then stir them together with the eggs in a large bowl. (If adding any extra fillings, cook them until tender then remove from skillet, drain on paper towels and add to egg and potato mixture.)
Leave about 2 tablespoons oil in the skillet, or add more if necessary so that 2 tablespoons remain. Heat over medium high heat until very hot. Pour in egg mixture, rotating the skillet until it's evenly distributed. Lower the heat to medium and cook, moving the skillet constantly, until the tortilla is light golden underneath.
Slide the tortilla onto a plate and heat 1 more tablespoon oil in skillet. Flip tortilla over and return it to the skillet, golden side up. Continue cooking until browned underneath but still just moist inside. Transfer to a cutting board and set aside to let cool slightly. Cut into wedges and serve warm or at room temperature, garnished with parsley.