Melt half the butter in a large skillet. Add onions and celery. Cook for a few minutes. Add flour and cook for 1 minute. Whisk in the stock.
Fold in chicken and veggies. Simmer for 2-3 minutes. Transfer to a baking dish. Add one piece of phyllo dough and brush it with melted butter.
Add the second piece of the phyllo dough, rotate it clockwise, and brush it with butter. Repeat layers until you have used all the phyllo dough.
Bake the pot pie for 20 minutes or until the top is golden brown. Cool the pot pie for 10 minutes before serving