In 3-quart saucepan, heat oil over medium heat. Add onion and celery; cook 2 to 3 minutes, stirring occasionally, until tender.
Stir in potatoes, corn and broth. Heat to boiling. Reduce heat; simmer uncovered about 5 minutes or until potatoes are tender.
Add whole fish fillets. Cover; cook 5 to 7 minutes or until fish flakes easily with fork. In measuring cup, mix milk and cornstarch; stir into chowder. Cook, stirring constantly, until mixture boils and thickens.