How to Make It
In a large heavy saucepan, melt the butter in 2 teaspoons of the olive oil. Add the chopped leeks and onions and cook over moderate heat, stirring occasionally, until softened, about 10 minutes. Add the water and bring to a simmer. Cover and cook for 10 minutes. Add the potatoes, cover and cook until tender, about 15 minutes. Working in batches, puree the soup in a blender.
Return the soup to the saucepan. Stir in the milk and cream and cook, stirring occasionally, until bubbles appear around the edge, about 6 minutes. Season with salt and white pepper.
In a small skillet, heat the remaining 2 teaspoons of olive oil. Add the sliced leeks and cook over moderate heat, stirring, until tender, about 4 minutes. Ladle the soup into bowls, garnish with the leeks, chives and parsley and serve.