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.
Combine flour and butter with a pastry cutter, in a food processor, or by rubbing together with your fingers. Add shortening, and salt. Continue to combine adding 1/4 cup cold water. Gather into a ball and flatten into an 8-inch disc. Refrigerate 30 minutes.