-Preheat oven to 400 degrees.
-Lightly grease a 1.5 qt round baking dish. (You can use the paper wrapper from the butter to do the greasing)
-Have a large bowl and strainer ready.
-In a large pot, bring chicken stock to a boil.
-Add in all vegetables and cook until tender-crisp.
-Strain contents of pot, reserving stock in the large bowl and vegetables in strainer.
-In the pot, melt butter over medium heat.
-Whisk in the flour and while continuing to whisk, cook for 4 minutes or until a pale golden color is achieved.
-Whisk in the warm, reserved chicken stock until smooth and thick.
-Stir in lemon juice and seasonings. Taste and adjust as necessary.
-Add chicken and vegetables to sauce and stir so that ingredients are evenly distributed.
-Pour this mixture into the baking dish.
-Place a puff pastry sheet over the top of the dish and trim slightly (at the corners of the pastry).
-Brush the top of the pastry lightly with the beaten egg. You will not need all of the egg.
-Bake in the preheated oven until pastry is golden brown (about 20 minutes).
-Allow pot pie to sit for 5 minutes or so before serving.
-Enjoy!