Heat a large non-stick frying pan over medium heat. Cook the chorizo for 2 mins each side or until browned. Transfer to a bowl. Add the onion to the pan and cook, stirring, for 3 mins or until light golden and just tender. Transfer to a separate bowl.
Heat the oil in the pan over medium heat. Arrange half the potato, overlapping slightly, over the base of the pan to cover completely. Top with half the chorizo and half the onion. Repeat with remaining potato, chorizo and onion.
Whisk eggs and water in a large bowl. Season with salt and pepper. Pour into the pan. Reduce heat to medium-low. As the omelette sets, gently pull up the edge so any uncooked egg runs underneath. Repeat 2-3 times duriong cooking. Cook for 20-25 mins or until almost set.
Preheat grill on high. Cook under grill for 5-10 mins or until golden.