Heat your oven to 350°F (180°C)
Place a saucepan of generously salted water on a high heat until it boils. Reduce to simmer, add the potatoes and cook for 15 to 20 minutes until you can easily place a fork through them. (note 3)
Meanwhile, place a large frying pan on a medium heat and cook the spinach until wilted. You may need to do this in two batches.
Place the spinach into a colander once it has wilted, drain the excess moisture by gently squeezing it with your hands and set it aside.
Wipe the frying pan you used to wilt the spinach clean and place it on a medium heat. Add oil and fry the cod and haddock for a few minutes until it is cooked and set it aside. (note 4)
Add the shrimp and fry for 1-2 minutes until opaque. Add a little more oil to the pan if necessary. Set them aside with the cod and haddock when they are done.
Wipe the pan clean again and place it on a medium heat. Melt the butter, then stir in the flour and add the milk gradually and stir continually until a thick and creamy sauce is formed.
Stir in the fish stock and add the garlic clove, nutmeg, lemon juice and zest, chives, dill and a little black pepper to taste.
Stir the cooked fish, shrimp and spinach and leave the sauce on your stove's lowest heat while you prepare the remainder of the fish pie.
When the potatoes are cooked, drain them using a colander and return them to the pan.
Add the butter and milk and mash until the potatoes are smooth and fluffy. Add extra milk if necessary. Season with black pepper and salt to taste.(note 5)
Remove the garlic clove and spoon the sauce into the baking dish.
Spoon the mashed potato onto the top and sprinkle with grated cheese and breadcrumbs (if using).
Grind a little fresh black pepper on top (no more than ¼ tsp) and bake in the oven for approximately 15 minutes until you see the sauce start to bubble around the edges of the mash potato topping and the cheese is melted and golden.
Place the fish pie under the grill for a few minutes to get the topping extra brown and crispy if desired.