1) Set the microwave to HIGH and put the potatoes in cooking for 10 minutes.
2) Cut them longitudinally into two halves and scoop out the flesh until the potato shells are empty except for a rim of potato around the skin.
3) Set the grill to HIGH.
4) Take a bowl and mash the potato flesh along with the milk and horseradish sauce.
5) Add the mackerel and spring onions by folding them in.
6) Season it and place the mashed potato mixture back into their skins with a spoon.
7) Take a baking tray and place the potatoes in it.
8) Grill them for 5 minutes or until they become golden and are properly heated.