To make the soup, in a large saucepan over medium heat, warm the olive oil. Add the leeks and sauté until softened, 3 to 5 minutes. Add the broccoli and sauté, stirring frequently, until slightly softened, about 2 minutes more.
Add the stock and bring to a simmer. Cover partially and cook until the vegetables are tender when pierced with a sharp knife, 15 to 20 minutes. Remove from the heat.
In a blender or food processor, puree the soup in batches until smooth and return the soup to the pan. Alternatively, process with a stick blender in the pan until smooth. Reheat the soup gently over medium heat. Season with salt and white pepper. Ladle the soup into warmed bowls and garnish with the sour cream, croutons and chives. Serve immediately. Serves 4.