Pre-heat oven to 400°. Arrange individual oven-safe baking or meal prep dishes on a baking sheet.
In a pot, melt butter over medium high heat. Once butter is hot, add onion, celery, garlic, and fresh thyme. Cook and stir for about 5 minutes or until tender. Add flour and stir to coat veggies evenly. No powdery flour should remain visible.
Add chicken stock and cream of mushroom soup, whisking to ensure there are no clumps of flour. Bring to a boil for 1-2 minutes, then remove from heat and set aside to cool.
Unroll the puff pastry onto a lightly floured surface. Trim to fit the opening of your baking dish (I prefer square so I don't waste much pastry!). If you need to splice 2 pieces of pastry together, just lightly wet the edges and work the dough to smoosh it together. Crimp the edges of the pastry using your fingers, then cut an "X" in the middle of the prepared pastry squares to allow steam to escape.
Whisk the egg and water together to create an egg wash. Brush the top and edges of the pastry with egg wash for optimal browning. (Note: Skip the egg wash if you plan to freeze the pot pies.)
Mix frozen veggies and rotisserie chicken into the soup mixture. Divide among 6 baking dishes (about 2 cups filling per serving).
Gently lay each pastry square on top of the filling. Push the corners down slightly if they are sticking up above the edge of the dish, but do not press down farther than that. This will help keep the soup mixture under the pastry as much as possible.
Bake at 400° for 35 minutes, then increase the temperature to 450° and move to the top rack of the oven for the last 10 minutes. The pastry should puff up slightly, become flaky on the top, and develop a deep golden brown color.
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FREEZING AND BAKING FROM FROZEN: