Skin the mackerel and flake into a bowl, removing any bones. Heat the grill to high.
In a large non-stick frying pan, heat the olive oil and gently fry the onions until soft and slightly coloured. Add the mackerel and mix with the onions until warmed through. Add the parsley, if using.
In a bowl, lightly beat the eggs and season with salt and pepper. Add the eggs to the pan and mix through with the onion and mackerel. Turn down the heat and leave to cook until the egg has just started to set.
Place under the grill to finish cooking – this will only take a few minutes if the grill is hot.
Leave to rest in the pan for a few minutes before serving.
Serve with a tomato and onion salad