Step 1 Preheat oven to 200°C. Steam potatoes until very tender. Mash with milk and oil until smooth. Cover and keep warm.
Step 2 Meanwhile, spray a large frying pan with oil. Cook onion and celery for 5 minutes over medium heat, until softened. Add garlic and tomatoes and bring to the boil. Reduce heat and simmer, covered, for 20 minutes.
Step 3 While tomato mixture simmers, place fish on a baking tray lined with foil. Bake for 15 minutes, until cooked. Roughly flake fish, combine with raw prawns and place into an 8-cup capacity ovenproof dish.
Step 4 In a small bowl, combine cornflour with 2 tablespoons water. Add to tomato mixture and cook, stirring, until mixture thickens. Stir through peas. Spoon tomato sauce over fish and prawns, and carefully combine.
Step 5 Gently spoon mash over fish and spread evenly to cover. Sprinkle with cheese and bake for 15–20 minutes. Serve with salad.