Step 1 Add the knob of butter to a large saucepan over a medium heat. When it has melted, add the onion and garlic. Reduce the heat and allow to gently sauté for 5 minutes. Add the potato and put the lid on the pan so it gently sweats for 10 further minutes.

Step 2 Stir in the watercress and pour in the stock. Blend in a food processor or with a stick blender until velvety smooth. Stir through the horseradish sauce and season generously with salt and pepper. Bring the soup back up to temperature before serving.

Step 3 Ladle the soup into bowls and flake generous hunks of smoked mackerel across the centre of each serving. Finally, swirl over a spoonful of horseradish sauce, olive oil, crispy onions and flakes of sea salt.