Thaw one sheet of puff pastry if using frozen pastry.
Melt butter in a large skillet. Add diced onions and celery and cook a few minutes on medium-low heat until softened, stirring often.
Add ¼ cup flour and cook stirring for 2-3 minutes.
Now add 1 cup chicken broth and whisk together cooking at low heat until thickened. Now add 1 cup milk and keep whisking, for a few more minutes.
Once the sauce is thick and smooth, add frozen peas and carrots, salt, garlic powder, thyme leaves and black pepper. Stir and cook until hot and bubbly.
Now stir in cooked turkey meat.
Transfer the mixture to a pie or casserole dish and let cool for at least 15 minutes.
While the pie filling is cooling, preheat the oven to 400 degrees Fahrenheit.
Place a sheet of cold puff pastry on top of the pie. If your casserole or pie dish is larger than your sheet of pastry, roll it out a little bit. For a round pie dish, cut off any hanging edges. Press the pastry to the edges of your baking dish.
In a small bowl whisk together egg yolk and cold water.
Brush with the egg yolk wash and make 2 venting slits in the middle.
Bake in preheated oven for 30-35 minutes until the pastry is puffed up and browned.
Let sit for 5-10 minutes before digging in.