In a large saucepan, sauté onion, celery and garlic in butter until tender. Add flour, sugar, salt, basil, pepper and broth; bring to a boil. Cook and stir for 1 minute; reduce heat. Add peas and cook for 5 minutes, stirring constantly. Stir in chicken. Pour into a greased 13x9 inch 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℉ (180℃) for 30 minutes. Cover and bake 10 minutes more or until the dumplings are done.