First make the filling, put the onions and steak in a large pan. Add enough water to just cover them and bring to a simmer. Cover with a lid, reduce the heat and simmer gently for 1½ hours.
Add the potatoes to the pan, along with some salt and pepper, and cook for 30-35 minutes more or until the potatoes are soft and the meat tender. The gravy should be nicely thickened by the potatoes.
Check the seasoning and add salt and pepper to taste.
Pour off 300-600ml/10-20fl oz of liquid from the pan – enough to leave the filling nicely moist but not swimming in liquid – and save this to use as gravy with the pie. Transfer the filling to a 1.2 litre/2 pint pie dish and leave to cool completely.
Heat your oven to 200C/400F/Gas 6.
For the suet crust, combine the flour and suet in a large bowl with some salt and pepper. Gradually mix in enough cold water to form a soft, slightly sticky dough. Leave to stand for five minutes.
On a lightly floured surface, roll out the pastry to a 7-8mm/1/3in thickness. Cut a 2cm/¾in strip of pastry. Dampen the edges of the rim of the pie dish with water. Stick the pastry strip onto the rim and dampen this too. Lay the remaining pastry on top. Press down to seal the edges, crimp or flute the edges of the pastry then trim off the excess.
Bake the pie for 30-40 minutes, or until the pastry is golden-brown. Leave to stand for 10–15 minutes before serving.