Heat oil in a large saucepan over medium-low heat. Add leek and garlic. Cook, covered, stirring occasionally, for 8 to 10 minutes or until softened. Increase heat to medium-high. Add potato. Stir to coat.
Add stock and 3 cups cold water. Cover. Bring to the boil. Reduce heat to medium-low. Simmer, partially covered, for 20 minutes or until potato is tender. Set aside for 5 minutes to cool slightly.
Blend, in batches, until smooth. Return to pan over low heat. Cook, stirring, for 2 minutes or until heated through. Serve sprinkled with chives.