How to Make It
Melt butter with olive oil in a large, heavy-bottomed saucepan over medium. Add leeks; cook, stirring often, until softened, 5 to 8 minutes. Add celery; cook, stirring often, until tender and deeply saturated in color, about 8 minutes. Add potatoes; stir to coat potatoes in butter and oil. Reduce heat to low, and cook, stirring often, until potatoes are coated in a shiny, milky starch, about 5 minutes. Stir in 7 cups water and chicken stock base; bring to a boil over high. Reduce heat to medium, and simmer, stirring occasionally, until potatoes are just tender and vegetables are softened, about 15 minutes. Remove from heat. Using a ladle, remove 2 cups of cooking stock (liquid only), and reserve in a small bowl. Let remaining soup stand at room temperature 30 minutes.
Stir milk, cream, salt, pepper, and nutmeg into soup. Stir in reserved stock, a little at a time, to reach desired consistency. (If desired, use a potato masher to mash some of the soup. To serve pureed, pour soup and reserved 2 cups stock, in 2 batches, into a blender. Process until smooth, about 20 seconds per batch.) Divide soup evenly among 8 bowls, and garnish with celery leaves, chives, and crushed potato chips.