Make the dough:

1. Place the flour, Crisco vegetable shortening, baking powder and salt in a large bowl. Using your fingers, rub the shortening into the flour until it resembles a coarse sand-like texture.
2. Add the water little by little and mix well with a knife to bring the dough together. The dough is ready when it is not too wet or sticky (if it is, add some flour).
3. Press the dough into a ball, wrap in plastic wrap and place in the refrigerator to rest for at least 30 minutes.

Make the filling:

1. Mix together the mustard and grated cheese. Put in the refrigerator while you cook the potatoes.
2. Peel the potatoes and cut them into small cubes. Place the potatoes in a pan of rapidly boiling, lightly salted water and cook until just tender (about 10 minutes). Meanwhile, peel the onions and cut each onion into 6 wedges.
3. Once the potatoes are just soft, add the onions and cook until the potatoes are soft (but not falling apart), about 8 more minutes.
4. Preheat the BBQ indirectly with pizza stone to 200 C.
5. Drain the potatoes and onions in a colander and return them to the pan. Add the tablespoon of butter and a splash of milk. Crush the potatoes and onions lightly with a fork and mix with the butter and milk to make a coarse stuffing. Add the cheese-mustard mixture and stir well.

Gather and bake the patties:

Roll out the dough and use a plate as a template to cut out two circles. Spoon the filling on one side of the circle.

Add a pinch of salt to the beaten egg to make the egg wash. Using a pastry brush, coat the perimeter of the dough circle with the egg wash. Fold the dough over it, pressing the edges together and crimping the sides of the patties.

Place the patties on a floured pizza stone and bake for about 40 minutes in the preheated BBQ, or until they reach an internal temperature of 75 C