Parboil potatoes, celery and carrot until fork-tender. Drain and season with salt and pepper. Layer vegetables and oysters in baking pan. Dot top with butter; pour heated milk over. Cover with pastry. Paint with egg wash. Make a few small slashes in the pastry for the steam to escape. Bake in a preheated 375 degree oven for 35 minutes or until crust is golden brown.