In an very large bowl, combine sugar, cinnamon, and nutmeg. Add apple slices; toss to coat.
In another very large bowl, mix the flour and 1-1/2 teaspoons salt. With a pastry blender, cut in the shortening until pieces are the size of small peas. In a small bowl, combine eggs, water, and lemon juice. Add to flour mixture; mix until moistened. Divide in half.
On a lightly floured surface, roll half of the dough into a 16x11-inch rectangle. Ease pastry into a 15x10x1-inch baking pan (should be a 1/2-inch pastry border around pan).
Drain apple mixture, if necessary. Arrange apple mixture evenly over the pastry in the pan. Dot with butter or margarine. Roll out remaining dough to a 15-1/2x10-1/2-inch rectangle. Fit pastry over apple mixture. Seal and crimp pastry edges together. Cut several slits in top pastry.
To prevent overbrowning, cover edge of pastry with foil. Bake in a 375 degree F oven for 25 minutes. Remove foil. Bake for 15 to 20 minutes more or until golden. Drizzle icing over hot pastry. Cool. To serve, cut pastry into squares. Makes 12 to 16 servings.
In a bowl, combine powdered sugar and milk.