Cook the potatoes in a pan of boiling water until tender. Add the shredded cabbage for the last 2 minutes of cooking. Squeeze lemon juice over the flesh of the mackerel fillets. Heat the grill and cook the mackerel fillet for 4-5 minutes on either side.
Meanwhile, mash the potatoes with the milk, olive oil, creamed horseradish and seasoning.
Divide the horseradish mash between four plates or shallow bowls and serve the mackerel fillets on top with any juices from the grill pan poured over, and sprinkled with snipped chives.
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