Boil the kettle. Put the sliced potatoes and green beans in a saucepan and pour over the boiling water. Cover, then
bring back to the boil and cook for a further 4-5 minutes.
Heat the oil in a frying pan over a high heat. Season both sides of the mackerel and put the fillets skin down in the pan. Cook for 2-3 minutes, then turn over and cook for another 1-2 minutes. Meanwhile, chop the spring onions and capers.
Drain the potatoes and beans and put in a mixing bowl together with the onions, capers, oil and mustard. Mix well to cover the potatoes and beans completely. Cut the lemonin half and squeeze over the juice. Season and serve with the mackerel.