Heat a large non-stick frying pan over a medium-high heat. Tip the flour, salt and a good grinding of black pepper in a large bowl. Add the lamb and toss well together until the meat is lightly coated with a dusting of the seasoned flour.
Add 2 tbsp of the oil to the hot pan, then cook the lamb pieces in two batches, turning every now and then until golden brown on all sides. As the lamb is browned, transfer using tongs or a slotted spoon to a large, heavy based saucepan or flame-proof casserole dish and set aside.
Once the lamb is browned, the onions can be added to the frying pan and cooked in the remaining fat until nicely browned, stirring regularly. Add a little extra oil if they seem too dry.
Tip the onions over the lamb, add the nutmeg, allspice and rosemary. Pour over the stock and add an extra 300ml of cold water. Bring the liquid to the boil, then reduce the heat and leave to simmer for 40 minutes or until the lamb is just tender and the liquid has reduced to a good gravy consistency. Season to taste then remove the pan from the heat, cover and leave to cool for 30 minutes.
While the lamb is cooking, make the pastry. Rub the fat into the flour and salt in a large bowl until the mixture resembles fine breadcrumbs. (You can also use a food processor.) Beat the egg lightly with the water and put 2 tbsp of it into separate bowl to use for the pastry glaze.
Make a well in the centre of the flour and add the egg. Stir with a wooden spoon until the dough comes together, then form into a ball and transfer to a floured surface. Lightly knead the dough into a ball, flatten slightly into a round and wrap in cling film. Place in the fridge to rest for 30 minutes.
When the pie filling has cooled to room temperature, preheat the oven to 200C/400F/Gas 6.
Toss the apples with the chopped sage and a little ground black pepper. Spoon half of the lamb mixture into a 2.2lt pie dish. Top with half the apples and cover with another layer of lamb. Finish with a final layer of apple, piling up above the edge of the pie. Ease a pie funnel into the centre of the pie to help support the pastry and set aside.
Unwrap the pastry and roll out into a large oval shape roughly 5cm larger than the pie dish. Cut a 2cm wide ring of pastry from around the edge of this oval. Brush the rim of the pie dish with the reserved egg mixture. Press the pastry ring onto the rim of the pie dish, overlapping slightly at the end join. Brush lightly with a little more egg and then cover the pie completely with the rolled pastry, making a small hole for the top of the pie funnel.
Press the edges firmly to seal then trim the pastry and flute all the way around. Brush the pastry with the remaining egg and put on a baking tray. Place in the centre of the oven and bake for 25 minutes or until the pastry is golden brown.
Serve the pie piping hot with creamy mashed potatoes and seasonal vegetables. Lightly cooked spinach goes very well.