Heat 1 tablespoon of olive oil in a large saucepan over medium heat. Add the sliced leeks and minced garlic, sautéing for about 2 minutes until the leeks soften and become translucent.

Incorporate the sliced button mushrooms, dried mixed herbs, salt, and pepper into the saucepan. Stir the mixture thoroughly, then cover and cook for approximately 8 minutes.

In a separate small saucepan, melt 1 tablespoon of dairy-free butter over medium-low heat. Whisk in 2 tablespoons of flour and gradually pour in 350 ml of dairy-free milk, stirring until the mixture is smooth and thickened, about 4-5 minutes. Add in ground nutmeg, salt, and pepper.

Remove the sauce from heat and mix in the sautéed leeks and mushrooms. Allow the filling to cool completely for about 10-15 minutes.

Preheat your oven to 200°C (400°F). Take the pastry out of the fridge, let it sit at room temperature for about 10 minutes, then roll it out gently.

Pour the cooled filling into your prepared pie dish, smoothing it out evenly. Place the rolled-out puff pastry over the top, trimming any excess overhang.

Use a knife to score the top of the pastry with a few slashes. Brush the surface with 4 tablespoons of aquafaba or dairy-free milk. Bake for 25 minutes or until golden brown.

Once baked, let the pie cool for a few minutes before slicing. Serve warm.