In a large saucepan, saute onion, celery and garlic in butter until tender. Add flour, sugar, salt, basil, pepper and broth; bring to a boil. Cook and stir for one minute; reduce heat. Add peas and cook for 5 minutes, stirring constantly. Stir in cubed chicken. Pour into a greased 13-in. X 9-in. X 2-in. Baking dish. For dumplings, combine biscuit mix and basil in a bowl. Stir in milk with a fork until moistened. Drop by tablespoonfuls onto casserole (12 dumplings). Bake, uncovered, at 350 for 30 minutes. Cover and bake 10 minutes more or until dumplings are done.