Place the Jersey Royals into a saucepan of salted water, bring the water up to the boil and simmer until the potatoes are cooked. Remove from the water and place to one side.
Peel and finely chop the spring onions and add to the potatoes along with the chopped chives. Add the mayonnaise and gently fold this all together
In a frying pan add a few drops of vegetable oil and place the mackerel fillets skin side down and gently cook until golden brown.
Gently flip the fillets over to the other side, add a knob of butter and a squeeze of lemon juice and season with the salt. Remove from the pan and rest for a couple of minutes