Add potato chunks to a large pot, cover with water, and add salt. Bring to a boil, then reduce to a simmer and cook until tender, about 10-15 minutes.

Drain potatoes well, then return to the pot. Add the butter and mash until smooth.

Stir in the milk and salt, taste and adjust seasonings to your preference and set aside.

Preheat your oven to 350°F (180°C), or 160°C for a fan oven.

Add fish to a saucepan or skillet, followed by an onion, bay leaves, salt, and cover with milk.

Turn on the heat to low, and allow the fish to very gently simmer for 7 minutes.

In the last minute of poaching add the shrimp, and cook for one minute.

Remove all seafood from the poaching liquid to a casserole dish, discarding bay leaves and onion. Strain milk into a cup to use the same vessel to make the sauce.

Melt butter in the saucepan over medium heat. Add garlic and cook for 30 seconds, then stir in flour to create a roux, cooking for 2 minutes.

Gradually whisk in milk (poaching liquid), avoiding lumps. Cook until the sauce thickens (2-3 minutes).

Stir in chopped parsley, black pepper and nutmeg. Taste the sauce, and adjust the seasoning if desired. Make sure that the sauce is thick as it won’t thicken further when it’s cooked in the oven.

Pour the sauce over the seafood in the casserole dish, and gently spread it if needed.

Spoon the mashed potatoes over the top, spreading them out with a spatula to cover the mixture completely. Use a fork to create a pattern/rough edges. Sprinkle cheese, and bake in the preheated oven for 30 minutes or until the pie is golden brown.

Remove from the oven, allow to rest for 5 minutes then garnish with chopped fresh parsley or chives and serve.