Preheat your oven according to the puff pastry instructions, typically around 400°F (200°C).

Heat olive oil in a large pan over medium heat. Add sliced leeks and minced garlic, sautéing until they're soft and fragrant—about 5 minutes.

Add dried tarragon, chopped chestnut mushrooms, sea salt, and cracked black pepper. Cook until the mushrooms are tender and have released their juices, approximately 5-7 minutes.

Sprinkle in all-purpose flour, stirring continuously for 1-2 minutes to create a roux that will help thicken the filling.

Gradually pour in the vegetable broth while whisking to prevent lumps. Cook for another 3-5 minutes until the mixture thickens slightly.

Stir in the oat cream, sweet corn, white miso paste, lemon juice, and whole-grain mustard. Cook for a few more minutes until the filling is thick and creamy.

Mix in the shredded chicken and remove the pan from heat. Let it cool slightly while you prepare the pastry.

Roll out the puff pastry on a floured surface and fit one sheet into your greased baking dish, ensuring it covers the bottom and sides.

Pour the creamy filling into the pastry base, spreading it evenly.

Cover the filling with another layer of puff pastry, sealing the edges well. Make slits on top for steam to escape.

Brush the top with a mixture of plant-based milk and maple syrup for a beautiful golden finish.

Bake in the preheated oven for about 30-40 minutes until the top is golden brown and the filling is bubbly.