circle on plastic wrap; cover. chill 30 minutes.
to prepare filling, place potato in a shallow microwave-safe bowl; add 1 tablespoon water. lightly cover with wax paper. microwave at high 2 minutes or until crisp-tender, stirring once.
heat a medium saucepan over medium heat. coat pan with cooking spray. add leek, celery, and garlic; cook 4 minutes. add mushrooms; cook 3 minutes. lightly spoon 1/2 cup flour into a dry measuring cup; level with a knife. place flour in a medium bowl; add broth, stirring with a whisk. stir broth mixture into leek mixture; bring to a boil, stirring constantly. reduce heat, and simmer 8 minutes. stir in potato, chicken, and remaining 