Preheat oven to 375 degrees.

In a medium saucepan, add butter, celery, onion, mushrooms and thyme leaves. Heat over medium heat. Sauté until celery and onions begin to become translucent, about 5 minutes.

Add diced chicken to the saucepan, stirring occasionally until chicken is cooked through, about 10 minutes. Season with salt and pepper.

Next, add the flour to the chicken and vegetables, combine well. Cook for 5 minutes.

Add the chicken broth, heavy cream, diced potatoes and frozen vegetables. Remove pan from heat.

Prepare for oven: Add one of the refrigerated pie crust to the bottom of a deep pie dish or casserole dish. Add the chicken and vegetable filling. Cover the top of the pie with the other crust. Place a few slits in the crust center.

Beat 1 egg together with 1 tablespoon of water to use as an egg wash. Brush the top crust with the egg wash. Reserve the leftover egg wash to use on the individual puff pastry toppers.
Place the pie into the oven and bake for 1 hour, until hot and bubbly and the crust is a beautiful brown.

The puff pastry toppers are completely optional. They do add a special touch to the individual servings.
Let the puff pastry thaw as directed while the pot pie bakes. Each sheet is folded over creating 3 sections. When thawed, using a pastry wheel or pizza wheel, cut into three sections. Cut each section into strips, then cut the strips in half. Using 3-4 pieces for the vertical and horizontal pieces, weave them together like a basket. I weave them together on a baking sheet lined with parchment. It makes it easier than transferring them later. Brush each of the lattice toppers with the egg wash.
Bake the topper in a 400 degree oven, 5-8 minutes until golden brown on top.