Preheat the oven to its highest temperature. Make three diagonal cuts in each side of the mackerels, cutting through to the bone. Season with salt and a twist of pepper and brush each with butter. Heat a roasting tray on top of the stove with the olive oil, carefully placing the fish in it. Transfer the tray to the oven and roast for 8-10 minutes.
While roasting the mackerel, melt the knob of butter in a saucepan. Add the spring onions and cook them over a medium heat for 1-2 minutes until just beginning to soften. Sprinkle in the mackerel flakes, stirring to warm through before adding to the hot mash along with the horseradish cream.
Serve the roast mackerel and mashed potatoes with a wedge of lemon.
The horseradish cream can be replaced by English or Dijon mustard, adding a teaspoon at a time until your preferred strength is reached.