Preheat oven to 400 degrees.
Heat a saute pan over medium high heat with 1 tbsp oil and 1 tbsp. butter.
Add carrots, celery, onion, garlic, salt and pepper. Cook until softened.
Add flour and butter, if needed. Let flour cook.
Add milk and 1/2 c. chicken stock. Stir to combine.
Add remaining vegetables. Stir.
Bring to a boil and remove from heat.
Add chicken.
Put all ingredients into casserole dish.
Cover with puff pastry. Crimp edges so that it is secure.
Brush with egg wash. Cut slits in puff pastry.
Place on a cookie sheet and bake for about 15 minutes, or until golden brown.
Let stand for 5-10 minutes before serving.