2 tablespoons better or extra virgin olive oil
3 medium potatoes, peeled and cut into small cubes
3 leeks, white and light green parts only, well washed and sliced into thin rings
1/2 to 1 cup cream, sour cream, or yogurt
salt and freshly ground black pepper
4 cups vegetable stock or water

1. Put the butter or oil in a large, deep saucepan over medium heat.  When the butter melts or the oil is hot, add the vegetables.  Sprinkle with salt and pepper and cook, stirring, for 2 or 3 minutes
2.  Add the stock and cook until the vegetables are very tender, about 20 minutes.
3.  Carefully purée the soup in a blender (or with an immersion blender—far easier!) then return in to the pot.  Stir in 1/2 to 1 cup cream, sour cream, or yogurt.  (It can be reheated and served as a warm soup at this point, too).  Add salt and pepper as needed, garnish with minced chives, and serve. 

– How to Cook Everything Vegetarian by Mark Bittman