Sautee some chicken breasts cut into bite sized pieces in butter till half done. Remove and let cool.
Add chopped onion, leek, carrots, celery, parsnips to pan and let cook till onions are translucent. Remove veggies from pan, add a little more butter, a couple of tablespoons of flour and make a roux. After several minutes, add warm chicken stock and let it come to a simmer. It will get thicker.
Add all the veggies, chicken, frozen peas, or any other veggie, and mix together
Place the chicken veggie mix in a casserole and lay on top puff pastry . Brush egg wash on top of puff pastry and bake at 375 till the pastry is golden.