Preheat your oven to 400°F (200°C).
In a large skillet, melt the butter over medium heat. Add the flour, and whisk continuously for 1-2 minutes to create a roux.
Slowly whisk in the milk, ensuring there are no lumps. Cook until the sauce thickens, about 3-5 minutes.
Add the shrimp, scallops, mixed vegetables, salt, pepper, and Herbes de Provence to the sauce. Stir to combine and let cook for an additional 5 minutes.
Transfer the seafood mixture into a pie dish and top with the puff pastry, trimming any excess. Make small slits in the pastry to allow steam to escape.
Bake in the preheated oven for 25-30 minutes or until the pastry is golden brown and crisp.
Let the pot pie cool for a few minutes before serving. Enjoy!