Preheat your oven to 400°F (200°C). Grease your pie dish with olive oil or butter and set it on a baking sheet.

In a large skillet, melt the butter over medium heat. Add the onion, celery, and carrot. Cook, stirring often, until the onion is soft and translucent, about 5–7 minutes.

Add the garlic and cook for 30 seconds more until fragrant.

Sprinkle the flour over the vegetables and stir constantly for 1–2 minutes.

Gradually whisk in the fish stock, then the milk and heavy cream. Keep whisking to smooth any lumps and bring to a gentle simmer until it thickens into a creamy sauce (3–5 minutes).

Stir in the thyme, half the parsley, lemon zest, and lemon juice. Taste and season with salt and pepper.

Turn the heat to low. Add the seafood pieces and frozen peas to the sauce, gently folding to combine. Cook for 2–3 minutes.

Spoon the seafood filling into your prepared baking dish, spreading it evenly.

Unfold or roll the puff pastry to fit over your dish. Drape it over the filling and trim any excess, leaving a little overhang. Press the edges to seal and flute or crimp as you like. Cut a couple of small slits in the top to vent steam.

Brush the puff pastry with egg wash. Place the dish in the oven and bake for 25–30 minutes, until the pastry is puffed and deep golden.

Let the pot pie rest for 10 minutes before slicing. Sprinkle the remaining parsley on top.