Cream of Leek and Potato Soup- Súp Tỏi Tây và Khoai lang Tây

This is an iconic soup of Julia Child, a French classic. 

It can be served cold or hot. This is a soup that you can drink.  A similar soup is Vichyssoise which is invented in the USA at the Algonquin.

Make 4 servings
2 large or 3 medium russet potato (about 1¼1bs)
3-4 large leeks
4-5 cups water
1½ tsp salt
1/2 cup heavy cream
1/2 cup sour cream or cream fraiche (optional)
1/2 tsp ground white pepper
2 Tbs chopped fresh chives, cilantro for garnish (optional)

1. Trim off the root ends of leeks just above the base.  Cut off and discard the dark green fibrous tops and keep the white and pale tender greens about 6-7 inches long.  Start 1/2 inch from the root, split each leek lengthwise in halves and quarters.  Spreading the leaves apart, wash and rinse off all dirts.  Thinly slice from the pale tender part down toward the root ends.  Peel and dice potato.
2. Add potato, leek and 1 tsp salt in a large pot and pour water in just barely cover all ingredients.  Bring it to a boil over medium high heat and low the heat down to simmer, cover.  Cook until potato and leek very tender about 20 minutes.  Taste and add more salt if desired.
3. Scoop and transfer the solid  to a blender or a food processor.  Process and add more liquid to adjust the thickness.  Stir in heavy cream and pepper.  Remove from heat.
4. Ladle into bowls, top with a dollop of sour cream and chives if use and serve hot.  The soup can be prepare 1 day ahead, cover and chill in the fridge and serve cold.