Heat a large, non-stick frying pan over a high heat. Add the chorizo and stir for a couple of minutes, until most of the Chorizo oil has been released. Remove with a slotted spoon and set aside.
Reduce the heat, add the onions to the same pan and fry until clear and beginning to brown and well coated with the Chorizo oil, add the boiled potatoes & stir to get that Chorizo oil coating the spuds.
Add the tomatoes and Chorizo to the pan and then flake the mackerel fillets in and gently mix, stir in the parsley and serve with a mixed leaf salad.