1. Preheat the oven to Gas Mark 4, 180°C (350°F).
2. Rinse the potatoes well, cut a notch and rub with oil. Sprinkle with coarse salt. Place on a baking tray and put in the oven for 35-40 minutes (depending on the size of the potatoes).
3. Divide the mackerel into small pieces and mix with the sour cream, lime zest, capers and finely chopped scallion. Season with salt and pepper. Keep cool until further use.
4. When the potatoes are cooked, cut in half, careful not to cut completely and add a knob of butter.
5. Divide the mackerel mixture between the two halves of the potato, finish with cress or finely chopped radicchio.
6. Season with freshly ground pepper and serve immediately.

Tip: If you use fresh mackerel, sear it well in a hot pan with a little oil, but make sure it is still pink inside. Cut into slices and cover the baked potato and top with the sour cream.