Preheat your oven to 400°F (200°C). Lightly grease a 9x13" casserole dish and set aside.

Melt the butter in a large skillet or Dutch oven over medium heat. Add the chopped onion and cook until softened, about 5-7 minutes. Stir in the minced garlic and cook for another minute until fragrant.

Sprinkle the flour over the onions and garlic, stirring constantly to cook for about one minute. This will cook out the raw flour taste.

Gradually whisk in the chicken broth until smooth, then whisk in the heavy cream. Bring the mixture to a simmer, stirring frequently, until it thickens. This should take about 5-7 minutes.

Stir in the garlic powder, onion powder, salt, pepper, and dried thyme. Add the frozen peas and carrots and the shredded chicken, stirring until everything is well combined and heated through.

Pour the chicken mixture evenly into the prepared casserole dish.

Unroll the puff pastry sheet on a lightly floured surface. Roll it out slightly, if needed, to fit the dimensions of the casserole dish. Carefully lay the puff pastry over the filling, tucking the edges slightly into the dish.

Brush the top of the puff pastry with the egg wash. Cut a few small slits in the pastry to allow steam to escape.

Bake for 30-35 minutes, or until the puff pastry is golden brown and puffed, and the filling is bubbly.

Let cool for a few minutes before garnishing with fresh parsley and serving. Enjoy!