Sift together two cups of the flour, the baking powder, and one teaspoon of the salt in a large mixing bowl. Add the shortening and use two knives or a pastry blender to cut the shortening into the dry ingredients. Add the milk and blend again to form a soft dough.

Turn the dough out onto a lightly floured surface and knead gently to finish blending. Roll out the dough into a 14-inch wide circle. Lift the dough and place it in the prepared skillet, letting the excess extend well over the sides.

Blend together the rhubarb, apples, sugar, remaining 1/3 cup of flour, mace, and remaining one-quarter teaspoon of salt in a mixing bowl. Pile the fruit mixture into the dough-lined skillet and dot with the butter.

Fold the excess dough toward the center of the pie, leaving the middle open and uncovered with dough.

Brush the top of the pie lightly with milk and sprinkle with sugar. Bake for 50 minutes, or until the fruit is tender. About 40 minutes into baking, cover the top of the pie loosely with aluminum foil so as to keep the top of the pie from becoming too dark.

Serve warm, topped with freshly whipped cream of vanilla ice cream, if desired. Serves 6 to 8