In small bowl, mix dressing ingredients until blended. Refrigerate until serving time.
Place apples in large bowl. Sprinkle with lemon juice; toss until coated.
Remove leaves from celery; chop leaves into 1/2-inch pieces to measure 1/2 cup. Cut celery stalks diagonally into 1/4- to 1/2-inch slices to measure 2 cups. Add to bowl with apples. Refrigerate until serving time.
Just before serving, toss apples and celery. Add dressing; toss until coated. Sprinkle with walnuts.