To prepare the crust, place flour, 1 teaspoon of salt, and baking powder in a mixing bowl. Sift to mix.
Put suet through the food-chopper. Rub the finely chopped suet through a fine sieve to remove the stringy parts. Then rub the suet into the flour and mix to a dough with 1/2 cup of cold water. Chop and fold for two minutes. Turn on a floured pastry board and divide into two pieces.
Roll out one-half of the dough until one-quarter inch thick and then turn a large plate over this dough and cut around the edge of the plate. Be sure that the plate is at least two inches larger than the top of the baking or casserole dish.
Now drain the oysters and look over carefully for the bits of shell. Place the oysters in a casserole or baking dish and add the stalk of celery that has been scraped clean and then diced and cooked until tender. Add onion, parsley, cream sauce, 1 1/2 teaspoons of salt, white pepper, and thyme. Mix thoroughly.
Make two or three small gashes in the top of the crust and cover the oysters with it, pressing the crust well against the edges of the dish. Brush the top of crust with water and bake in a moderate oven for thirty-five minutes.
Use equal parts of the oyster liquor and milk for making the cream sauce. Chop the celery leaves as well as the stalk.
Now roll out the balance of the pastry and cut into three-inch squares. Score the tops lightly with a knife or prick with a fork, and place on a baking sheet and bake a delicate light brown. Wrap in a napkin to keep warm.
When ready to serve the oyster pie, place two of the squares of pastry on a plate and then lift on the oyster pie, and then place a second piece right over the crust of pie. Pour over this top piece of pastry two tablespoons of the sauce from the oyster pie.