Heat oil in pan over medium heat and add potatoes, stirring to coat. Add salt and pepper. When sizzling add stock and stir. Cover and cook for 15-20 minutes or until the potatoes are very tender. Stir in butter or olive oil. Add chives, then potato mixture to a blender or food processor and blend until smooth. Add milk, stir and serve chilled, with fresh sliced chives as a garnish.