Heat the oil in a large saucepan. Add the leek and sauté for 3–4 minutes. Then stir in the potatoes, pour in the stock, bring to the boil and simmer for 12–15 minutes until the potatoes are tender. Stir in half the salmon chunks and cook for a further 2 minutes.
Add the yogurt and stir through. Transfer the soup in batches to a food processor or blender and process until smooth, transferring each successive batch to a clean saucepan.
Juice the horseradish. Pour the juice into the soup, add the remaining salmon chunks and heat through gently. Season with salt and pepper.
Ladle the soup into bowls and garnish with some chopped chives.