Boil potatoes in water to soften. While water heats and boils, begin step 3.
Place 2 tablespoons of the butter in a large skillet. Add the onion, celery, carrots, parsley, oregano, salt and pepper. Cook and stir until the vegetables are soft. Stir in the broth.
In a medium saucepan, melt the remaining 2 tablespoons butter. Stir in the chicken and flour. Add the milk, and heat through. Stir the chicken mixture into the vegetable mixture, and cook until thickened.
Pour the chicken and vegetable mixture into a baking dish such as a 2.5 quart Corningware dish.
Place the puff pastry sheet over the mixture, cutting the sheet as needed to cover adequately.
Whisk the egg white with 1 tbsp water and brush over the puff pastry.
Bake in the preheated oven for 20-25 minutes, or until the puff pastry is golden brown.