1. In a large saucepan or large, deep skillet, saute onion, celery, and garlic in butter until tender. 2. Add flour, sugar, salt, pepper, and broth; bring to a boil. Cook and stir for 1 minute; reduce heat. Add peas and cook for 5 minutes, stirring. Stir in the chicken. 3. Pour the mixture into a greased or sprayed 9x13-inch baking dish. 4. For the dumplings, combine biscuit mix and milk with fork until moistened. Drop by spoonfuls onto casserole, making 12 dumplings. 5. Bake, uncovered, in a preheated 350-degree oven for 30 minutes. Cover with foil and continue baking 10 minutes more.