Preheat oven to 400 F.
In a large pot, over medium heat add in butter. Once butter begins to bubble, add yellow onion, mushrooms, diced celery, shallot and fresh thyme. Cook until fragrant, stirring often. About 3 minutes. Add in minced garlic. Continue to stir and sautè veggies until softened.
Next, add in 2 tbsp of flour. Stir to coat the veggies. Then, slowly whisk in chicken broth. Stirring frequently until the liquid thickens. Season with onion powder, garlic powder, Italian seasoning, black pepper and salt to taste
Add in shredded chicken, and mixed vegetables. Allow to simmer then stir in the heavy cream.
In a greased cast iron skillet, add a sheet of puff pastry. Pop in the oven for 7-minutes until the puff pastry puffs up. Remove from the oven, then add the pot pie filling on top.
On a flat surface, unfold the second puff pastry and cut into even squares or rectangles. (I recommend doing this while the puff pastry is slightly cold)
Transfer pot pie mixture into the baking dish. Top with puff pastry squares making sure the top of the pie is covered.
Combine 1 egg yolk and 2 tbsp of water. Brush egg wash on top of the puff pastry. Bake in the oven for 30-40 minutes or until the crust is puffed and golden.
Remove from the oven, top fresh thyme. Allow the pot pie to cool before serving.