Preheat the oven to 400 degrees F. Add the butter to a medium saucepan over medium heat. Once melted, add the onion and cook for 2 minutes, stirring occasionally. Add the garlic and cook for 30 seconds, stirring constantly.
Whisk in the flour and cook for 30 seconds, continuing to whisk.
Gradually add the chicken broth and milk to the flour mixture, whisking constantly. Bring to a boil, and then let it boil for 2 minutes, whisking the whole time.
Remove from the heat. Whisk in the garlic powder, onion powder, salt, black pepper, and dried thyme.
Stir in the peas and carrots and chicken.
Transfer the chicken mixture to an 8x8-inch casserole dish.
If necessary, roll out the puff pastry so it fits across the top of the casserole dish with about ½-inch extra on all 4 sides. Place the puff pastry on top and cut 4 slits on top for steam to escape.
Whisk together the egg with the water. Brush a little of this egg wash on the puff pastry.
Bake until the pastry is golden, about 25 to 30 minutes. Sprinkle the parsley on top (optional) then serve.
See my notes above for freezing tips.
Use only frozen vegetables for this.
Do not forget to cut slits in the top of the puff pastry.
Don't skip the egg wash, this gives it its nice golden color.