Heat the olive oil in a large pot on medium heat and add the leeks. Sautée for 7 minutes or so until they have cooked down and taken on some color.
Add the garlic, bay leaves, rosemary, turmeric, onion powder, garlic powder, miso paste and 2 cups of almond milk. (Reserve the last 1/2 cup for later). Bring to a simmer, then turn the heat down to low and cover.
Simmer for 35 minutes. Then, add the remaining 1/2 cup almond milk, and raise heat slightly until it returns to a simmer. Lightly season the cod with salt and pepper, then add for 5 minutes.
Remove fish to a plate and flake apart slightly with two forks.
Add the peas and simmer for another 5 minutes.
Discard the bay leaves and rosemary.
Strain the mixture through a sieve into a large jug -- RESERVE THE INFUSED ALMOND MILK! You should be left with at least 1 cup of almond milk.
Preheat the oven to 375F / 190C. Take your pastry out of the fridge.
Wipe your pan clean and heat the vegan butter over a low heat. Once it has melted, stir in the flour using a whisk to form your roux. Whisk until the flour and butter have formed a paste that does not stick to the sides of the pan. Cook like this for 1-2 minutes to cook off any raw flour taste.
While whisking, pour in 1/2 cup of the reserved infused almond milk, and bring the mixture to a boil. Boil for 2 minutes, then reduce the heat to low, still whisking, until the roux starts to thicken.
When there are no more lumps, add the remaining 1/2 cup of infused almond milk and whisk until smooth.
Remove pan from the heat and stir in the vegetables. At this point, taste the mixture and season with additional salt if desired.
Pour the mixture into a deep bottomed casserole dish, add fish, and stir to combine. Lightly flour a work surface and roll out your pastry to size with your casserole dish. The pastry sheet should be slightly larger than the dish.
Lay puff pastry over the dish and press sides down with a fork.
Brush the top of the pastry with the melted vegan butter.
Cook in the oven for ~20-25 minutes or until the top is golden brown.