Potato Bisque

1/4 cup butter
1 large onion, chopped
1 cup chopped celery and leaves
4 cups 1/2-inch cubed potatoes
1/4 cup finely chopped parsley
1/2 teaspoon salt
1/4 teaspoon pepper
1 quart regular strength chicken broth or
    4 bouillon cubes plus 1 quart water
1 quart milk
3 tablespoons cornstarch
1/4 cup water

In an 8- to 10-quart kettle, melt butter over medium heat. Add onion and celery and saut