Preheat your oven to 400°F (200°C). Lightly butter or spray a 9-inch pie dish or a 2-quart baking dish. Keep the puff pastry chilled until ready to top.

In a large skillet over medium heat, melt the butter. Add the chopped onion, carrots, and celery. Cook, stirring occasionally, until vegetables are soft — about 6–8 minutes. Add garlic and cook for another 30 seconds until fragrant.

Sprinkle the flour over the vegetables and stir constantly for 1–2 minutes. This cooks off the raw flour taste. Slowly whisk in the stock until lumps are gone and the mixture thickens. Add the half-and-half and cream, whisking to combine. Bring the mixture to a low simmer.

Stir in the Dijon mustard, Old Bay (or substitute), salt, and pepper. Let the sauce simmer for 2–3 minutes until it’s glossy and coats the back of a spoon. Taste and adjust seasoning.

Fold in the mixed seafood and frozen peas. Cook gently for 3–4 minutes — you just want to heat the seafood through. If you’re using pre-cooked shrimp or crab, add them at the end to warm through and avoid becoming rubbery. Stir in the parsley.

Transfer the seafood mixture to the prepared baking dish. Top with the puff pastry sheet, tucking the edges down around the dish. Brush the pastry with egg wash and cut a few small slits for steam to escape.

Bake for 20–25 minutes, or until the pastry is puffed and golden brown and the filling is bubbling around the edges. If the pastry browns too quickly, loosely tent with foil.

Let the pot pie rest for 5–10 minutes before serving. Squeeze a little lemon over each portion if you like a bright finish.