- Preheat the oven to Gas Mark 4, 180°C (350°F).
- 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).
- 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.
- When the potatoes are cooked, cut in half, careful not to cut completely and add a knob of butter.
- Divide the mackerel mixture between the two halves of the potato, finish with cress or finely chopped radicchio.
- 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.