In a large skillet, heat 1 tablespoon of olive oil over medium heat. Add the sliced leeks and minced garlic, sautéing for about 2 minutes until they soften.

Next, add the sliced button mushrooms, 1 teaspoon of dried mixed herbs, a pinch of salt, and pepper. Stir and cover the skillet, cooking for about 8 minutes.

In a separate saucepan, melt 1 tablespoon of dairy-free butter over medium-low heat. Whisk in 2 tablespoons of flour for 1 minute to create a roux, then gradually pour in 350 ml of dairy-free milk.

Pour the creamy sauce into the skillet with sautéed leeks and mushrooms. Gently fold together until well combined and let it cool completely.

Preheat your oven to 200°C (392°F). Roll out the thawed vegan puff pastry if necessary, ensuring it covers the pie dish.

Pour the cooled creamy filling into a pie dish, spreading it evenly. Place the puff pastry over the filling, trimming excess edges.

Crimp the edges of the pastry with a fork and score the top to allow steam to escape. Brush the surface with 4 tablespoons of aquafaba.

Bake the assembled vegan pie in the preheated oven for about 25 minutes or until the pastry is puffed and golden brown.