Boil the potatoes in their skins for 10-15 minutes until tender.
Drain potatoes, allow to cool, and cut into quarters.
Fry the potatoes in olive oil for 7-8 minutes until golden and crisp.
Drain the fried potatoes on kitchen paper and set aside.
Preheat the grill to its highest setting.
Score the skin of each mackerel fillet three times on both sides.
Mix softened butter with spices, sugar, mustard, Tabasco, and vinegar.
Rub the spiced butter over the mackerel and grill for 4-5 minutes.
Mix potatoes with shallots, soured cream, horseradish, and chives.
Serve the mackerel immediately with the horseradish potatoes.