1Preheat your oven to 400°F (200°C).
In a large saucepan, melt the butter over medium heat.
Add the diced onions and cook until they are translucent.
Stir in the flour, salt, pepper, and thyme until well combined.
Gradually whisk in the chicken broth and milk, cooking and stirring until the mixture thickens and bubbles.
Add the cooked chicken, peas and carrots, and potatoes to the sauce. Stir well to combine.
Pour the chicken mixture into a 9-inch pie dish.
Roll out the puff pastry sheet and place it over the chicken mixture, trimming any excess.
Press the edges to seal and cut a few slits in the top to allow steam to escape.
Bake in the preheated oven for 30-35 minutes, or until the pastry is golden brown.
Let the pot pie cool for a few minutes before serving.