Prepare the Seafood: Poach the fish in 500ml of milk with a bay leaf over low heat for 5 minutes. Remove fish and flake it, discarding any skin. Retain the milk for the sauce.
Make the Sauce: In a pan, melt butter and sauté onions and celery until soft. Stir in flour and cook for 2 minutes. Gradually add the reserved milk, whisking continuously to avoid lumps. Add lemon juice, parsley, and wine if using. Simmer for 5 minutes.
Combine the Ingredients: Add flaked fish and prawns to the sauce, stirring gently to coat. Transfer to a baking dish.
Prepare the Mashed Potato Topping: Boil potatoes in salted water until tender. Drain, mash with warm milk and butter, then season with salt and pepper. Stir in half the cheddar cheese.
Assemble the Pie: Spread the mashed potatoes evenly over the seafood mixture. Sprinkle remaining cheese on top.
Bake: Preheat oven to 200°C / 390°F. Bake for 25-30 minutes, until golden and bubbling.
Serve and Enjoy: Let the pie rest for 5 minutes before serving. Pair with garden peas or a fresh salad.