Preheat oven to 430 degrees F (220 degrees C).
Heat the Olive Oil (to taste) and Butter (to taste) in a large saucepan over medium heat and fry the Yellow Onion (1) and Leek (4) until soft and translucent.
Stir through the Garlic (2 clove) and All-Purpose Flour (90 gram) to create a thickened paste (roux). Cook for a minute then pour in the Fish Stock (300 milliliter), whisking the mixture until smooth.
Slowly add in the Fresh Cream (250 milliliter) and juice of the Lemon (1), stirring to combine. The sauce needs to evenly coat the back of a spoon. When thickened to the right consistency, remove the sauce from the heat.
Stir through the White Fish Fillet (300 gram), Fresh Squid (150 gram), Fresh Mussels (150 gram), and Haddock Fillet (150 gram), then add in the Fresh Dill (4 sprig), discarding the stalks. Season with Coarse Sea Salt and Pepper (to taste) and transfer into a large ovenproof dish.
Roll out the Puff Pastry (400 gram) on a lightly floured surface to fit the pie dish. Gently cover the pie mixture with the pastry, pressing the edges down to seal in the filling. Use a pie bird or alternatively poke a few holes in the pastry to let steam escape.
Brush the pastry all over with Egg (1) and bake the pie in a preheated 430 degrees F (220 degrees C) oven until golden and flaky, about 10 to 15 minutes.
Serve the pie with a side of your choice, extra lemon wedges and a bottle of your favorite red wine.