1 tbsp freshly grated horseradish or bottled horseradish
Procedure
Using a sharp knife trim and discard any roots and tops, leaving just the green part that joins the main shank. Then cut each leek in half lengthways. Clean leeks under running water, keeping each leek intact.
Lay leeks into a heavy based wide saucepan, no thicker than 2 layers. Cover with water, season with salt and bring to a simmer over high heat. Maintain a simmer and cook until leeks are just tender, about 40 minutes. Using a slotted spoon, remove leeks to a tray and pat dry with a clean tea towel or paper towel. Allow leeks to cool.
Spoon 60ml of leek cooking water into a bowl, add sour cream, vinegar, mustard, horseradish and whisk to combine. Season to taste with sea salt and freshly ground black pepper.
To serve cut leeks crossways into 5-6cm pieces, place leeks on a plate and spoon over dressing.