If you haven't cooked your own corned beef, you can find some at the deli - but either way, make sure it's sliced super thin. I find my inexpensive hand slicer is ideal for making short work of slicing chilled meats.
Put the sauerkraut in a cast iron or other non-stick skillet on medium heat. For every cup of sauerkraut, add about 1/2 teaspoon of caraway seeds and several grinds of black pepper. Saute the sauerkraut until just begining to brown slightly and the moisture is absorbed. Add the corned beef and mix together well, breaking up the corned beef slices as you do until all is well incorporated. Turn off the heat, add 1 cup grated Swiss cheese for every two cups of corned beef/sauerkraut mix and stir well. Cool to room temperature.
Roll out your pastry dough - I used a store-bought dough from the refrigerated section, brought it to room temperature and rolled it out just slightly. You don't want it too thin - the filling is heavy and things could break apart. Cut circles from the dough using a glass, small bowl, pastry round -- whatever you have, and what ever size you'd like. You could make a mega-pie and eat with a fork, but hand-held is fun! Smear a bit of strong mustard on half of the dough circle and put on a good mound of the filling. Fold over, align the edges and crimp with a fork. Brush on a thin layer of egg wash (one egg, well beaten) and poke a couple of small air holes.
Bake in a 350 degree oven for 10-15 minutes (this depends on the size of your pie!) until golden brown. Serve with any or all of : Strong mustard, horseradish sauce, Russian dressing. Enjoy!