Cook the potato in boiling water for 8-10 mins or until tender. Drain and allow to steam-dry. Heat oil in an omelette pan, add chorizo and cook for 2 mins. Add the potato and cook for a further 5 mins until the potato starts to crisp. Spoon pan contents out, wipe pan and cook a 2 or 3-egg omelette in the same pan. When almost cooked, scatter with the chorizo and potato, parsley and cheese. Fold the omelette in the pan and cook for 1 min more to melt the cheese.