1. Preheat the oven to 220 °C.
2. Spray a 200 x 200 mm ovenproof dish with non-stick cooking spray.
3. Cut the chicken breasts into small pieces.
4. Pour the oil into a large frying pan over a medium heat. Add the onion, ham or bacon, chicken and garlic, and gently stir-fry until brown.
5. Add the mushrooms, courgettes, salt and pepper, and cook until just tender.
6. Mix the soup powder with the milk in a cup or mug. Add some of the boiling water to fill the cup and mix well. Pour over the chicken and mushroom mixture in the frying pan. Add the rest of the water and the oats, and stir until the sauce has thickened slightly.
7. Pour the pie filling into the greased ovenproof dish.
8. Take the room-temperature phyllo pastry sheets and loosely pile them in a ruffled heap on top of the chicken filling. Brush lightly with the beaten egg.
9. Turn the oven down to 180 °C and bake for 15–20 minutes, until the pastry is golden and the filling is heated through.
Serve with a large tossed salad or cooked vegetables.