½ pointed cabbage, leaves cut into 3cm/1¼in squares
4 tbsp butter
1 bunch chives, snipped
1 tbsp chopped red onion
2 tbsp capers
3 tbsp wholegrain mustard
4 tbsp soured cream
1 bunch parsley, chopped
2 tbsp grated fresh horseradish
1 lemon, cut into wedges
Bring the fish to room temperature for about 30 minutes. Season the mackerel with salt lightly on both sides and set aside.
Bring a large saucepan of lightly salted water to the boil and add the potatoes. Cook for about 15 minutes or until tender. Drain, return to the pan and cover to steam slightly.
Meanwhile, bring a saucepan of water to the boil and add the eggs. Cook for 6 minutes then carefully remove and put into iced water to cool down. Peel and cut the eggs in half.
Blanch the cabbage in lightly salted boiling water for 20 seconds. Remove the cabbage with a slotted spoon and drain on kitchen paper.
Fry the butter in a saucepan over a medium heat and stir until it is golden-brown and smells nutty. Pour into a heat-resistant bowl.
Place the cooked potatoes in a large bowl. Crush the potatoes with a fork or masher and combine with the blanched cabbage, browned butter, chives, red onion, capers and mustard. Top with the soured cream and parsley and horseradish.
Place a frying pan over a high heat. Wipe the mackerel fillets on both sides and grill for 3–4 minutes, skin-side down. Turn and cook for 2–4 minutes on the other side, until cooked through.
Serve the mackerel with the crushed potatoes, eggs and lemon wedges on the side.