Soured Cream, Smoked Mackerel and Horseradish

• 4 large, floury potatoes
• 2 egg yolks
• 4 tablespoons soured cream

Bake the potatoes until tender, then cut a lid off each one. Carefully scoop out the flesh into a bowl and mash until smooth. Beat in the egg yolks, soured cream and horseradish. Flake the smoked mackerel into pieces and mix with the potato, then add the rocket leaves and season with salt and pepper. Fill the potato shells with the mixture and return to the ov