Pierce the potato several times all over with a fork. To oven cook: preheat the oven to 200ºC/fan 180ºC/gas mark 6 and cook for 1 hour or until soft and fluffy. To microwave: place the potato on a microwavable plate and microwave for 10-15 minutes until soft.
While the potato is baking, prepare the filling. Remove the skin and flake the smoked mackerel flesh into a bowl. Break it down with a fork, the more you mix, the smoother the topping will be. If you prefer a coarser type, go easy with the fork. Add the crème fraîche and lemon juice and mix it up. If your mackerel wasn’t peppered, you might like to add some but it should be salty enough.
Cut open the potato and top with the creamy mackerel, garnished with the parsley, if using, and serve with salad.