Preheat the oven to 180°C.
In a pan add all the seafood, pour milk until it is immersed. Boil for 8 to 10 minutes.
Drain into a bowl and keep aside both the fish/ prawns and milk. Don't throw the milk.
Cut potatoes into big chunks, boil them and keep aside.
In another pan, add butter, once it melts, add the chopped garlic, onion and leek and sauté until translucent. Add the drained milk and cook for 2-3 minutes. Mix corn flour in a tablespoon of water and add to the pan, stirring continuously. Add the chopped tarragon, salt and pepper, switch off the flame.
Prepare mashed potatoes using the boiled potatoes - add 2 tablespoons butter, salt, pepper and 1/4 cup milk and mash everything up well to make mashed potatoes.
Arrange everything in an oven-safe dish. First, arrange fish and prawns at the bottom, pour the sauce over it, pipe the mashed potatoes on top and make impressions with a fork and bake for 20 minutes.
