Fry the onions in the butter until translucent, then add the apple, chicken, currants and chili powder.
Cook for about a minute, then add the tomatoes and salt to taste. Keep cooking for another five minutes or so (the finished mixture should be pretty dry).
Meanwhile, make the dough. Sift the flour and baking soda together, then add the butter, working it with your fingers until you get a texture like fine breadcrumbs.
Now whisk the egg whites together with one egg yolk and add it to the dough. Add the water in small amounts until you get the right consistency for a pastry dough. Cut out circles about 6 inches wide or so.
Drop enough filling on each dough piece to cover about half the circle, with enough room left over to pinch the edges together.
Fold over to a half-moon shape. Repeat until you either run out of dough or filling. Prick each pie to vent.
Mix the remaining egg yolk with 1 tbsp of water and brush the surface of each pie.
Transfer to a well-oiled baking sheet (they stick, so make sure to use plenty of grease) and bake at 350 degrees for 30 to 35 minutes, or until the pies turn a nice golden brown color.