Place the vegetables and whole pieces of fish in a pan, excluding the prawns
Cover with milk and add a couple of stems of parsley, pepper, salt, and bay leaf
Gently heat the mixture until the milk comes to a boil, then turn off the heat, cover, and leave for 15 minutes
Pour the mixture into a colander, reserving the milk
Peel and boil the potatoes, then mash them using a little of the reserved milk and butter
Remove the skin and any bones from the fish, flake it, and place it in a greased casserole dish with the vegetables and prawns
Melt butter in a saucepan, add flour until absorbed, and then slowly add the reserved milk, stirring constantly
Stir in the parsley and check the seasoning, adding more salt and pepper if needed
Pour the sauce over the fish and vegetables, then top with mashed potato
Optionally, sprinkle cheese on top
Bake at 200°C (400°F) for 20-30 minutes or until the topping begins to brown