Preheat the oven to 375F. Coat a 9-by-13-inch baking dish with cooking spray.
In a large pan, over medium-high heat, add cooking spray to coat. Add the onion, celery, and garlic to the pan, along with 1/2 tsp of salt and 1/2 tsp of black pepper. Cook until starting to soften, about 3 minutes.
In a large measuring cup, whisk together 1 1/2 cups chicken stock, half and half, and cornstarch. Add this mixture to the pan and stir for a minute or so. Add in thyme, sage, and remaining salt and pepper.
Reduce heat to medium and stir in cream cheese until melted. Fold in chicken and defrosted vegetables.
Bring everything to a simmer and let it cook for about 3-4 minutes. Splash in more stock if the mixture gets too thick, but you do want it to have a thicker gravy consistency. Continue to simmer if mixture it too soupy, until it gets to a thicker gravy consistency.
Once everything is heated through and mixture is at the consistency you like, pour it into the baking dish and spread evenly.
Carefully unfold the puff pastry and roll it out on a cutting board to fit your baking dish. Lay it on top of the filling and cut some slits in it to let steam release as it cooks.
Brush the top of the pastry with the egg wash. Bake for 40-45 minutes, or until the crush is golden. Let rest for 10 minutes. Cut into 8 portions and serve.