Wash, dry and dice the chicken fillet into bite-sized cubes. Combine them with the corn flour, soy sauce and black pepper and set aside to marinate for 15 minutes. The corn flour will help to keep the chicken tender while the soy sauce will help the chicken to brown better.
Heat a large non-stick pan (deep enough to hold liquid) over high heat and add in some oil. Turn the heat down to medium and sear the chicken pieces until browned on all sides. Do this in a few batches (adding more oil if necessary) if your pan cannot hold all the chicken pieces in a single layer. Remove the seared chicken (they don't have to be fully cooked) with a slotted spoon and place them on a large plate.
Keeping the pan over medium heat, add in the sliced mushrooms and sauté until the mushrooms have reduced in size. Season with a little salt and black pepper. Add in the seared chicken and the potato cubes. Add in the chicken stock, flour and butter and turn the heat up to high and bring the mixture to a boil. Add in the dried sage and cook until the mixture has thickened. Stir in the sour cream and turn off the heat.
Transfer mixture to a 8x8-inch greased baking tray. Cover with puff pastry / shortcrust pastry, and make decorative patterns on the edges if desired. Cut 3 slits to release steam
Brush the top with the beaten egg and bake for 20 minutes, until the pastry is golden-brown and the chicken juices are bubbling and overflowing.
Let the chicken pie rest for 5 to 10 minutes before serving.
NOTES
To serve in single-serving pots, after Step 4, divide chicken mixture among 4 ramekins. Cut the pastry into 4 squares and cover the ramekins, stretching slightly if necessary. Cut a slit on each pastry and brush with egg. Bake for 15 to 20 minutes.