In a large bowl, combine the flour, 1/2 teaspoon salt and baking powder. Cut in shortening until mixture resembles coarse crumbs. In a small bowl, whisk the egg yolks, water and lemon juice; gradually add to flour mixture, tossing with a fork until dough forms a ball. Chill for 30 minutes.
Roll out dough between two large sheets of waxed paper into a 17x12-in. rectangle. Transfer to an ungreased 15x10x1-in. baking pan. Press pastry onto the bottom and up the sides of pan; trim pastry even with top edges. Bake at 325° for 10 minutes.
Meanwhile, in a large saucepan, combine the sugar, cornstarch and remaining salt. Add cream; stir until smooth. Cook and stir over medium heat until mixture comes to a boil. Cook and stir 1-2 minutes longer or until thickened. Remove from the heat; stir in butter, vanilla and nutmeg. Pour into crust. Sprinkle with additional nutmeg.
Bake for 40-45 minutes or until a knife inserted in the center comes out clean and crust is golden brown. Cool on a wire rack. Cover and refrigerate for at least 1 hour before cutting. Refrigerate leftovers.