Potato, Chicken, and Coconut Soup

Potato, Chicken, and Coconut Soup(Adapted from CL)

2 1/2 cups chicken stock
2 1/2 cups water
2 cups chopped peeled baking potato
1 cup chopped onion
1 cup chopped celery
1 cup chopped carrot
2 tablespoons minced peeled fresh ginger
1/2 teaspoon salt
2 (5-inch) stalks lemongrass, cut in half lengthwise
3 garlic cloves, minced
2 cups shredded cooked chicken breast
3 tablespoons chopped fresh cilantro
4 teaspoons fish sauce
1 tablespoon fresh lime juice
1/2 teaspoon chile paste with garlic
1 13.5-ounce can light coconut milk

Combine first 9 ingredients in a large saucepan and bring to a boil. Reduce heat; simmer 25 minutes or until vegetables are tender. Discard lemongrass. Cool slightly. Pour 2 cups potato mixture in a blender; process until blended. Pour into a bowl. Repeat procedure with 2 cups potato mixture. Return pureed mixture to pan. Stir in chicken and remaining ingredients; cook over medium heat 5 minutes or until thoroughly heated.