Trim stem end from celery. Remove leaves, tie with a string into a bundle; set aside. Separate stalks into ribs; thinly slice enough to make 3 cups. In large saucepan melt butter. Add onion and curry powder. Cook, stir until onion is transparent. Add chicken broth, salt, white pepper and re- served celery leaves. Bring to a boil. Reduce heat; simmer covered, for 15 minutes. Add reserved celery slices. Simmer until barely crisp tender, about 10 minutes. Remove celery leaves. Add apple; simmer for 1 minute. Mix flour mixture and water; blend into saucepan along with milk. Cook, stir until slightly thickened, about 3 minutes. Stir in chicken, heat until hot.