For the crust: measure the flour, baking powder and salt into a bowl and stir to combine. Add the shortening (or suet if you have it) and rub together until mixture resembles coarse sand. Add enough cold water for the dough to come together. Take out 2/3 of the dough and roll out on a lightly floured surface until 1/8-inch thick. Use this dough to line the pudding basin, pressing dough into the curves of the basin and filling any gaps using cold water and extra dough. Trim dough flush with top of basin. Set the remaining dough aside for the lid.
For the filling: use the sausage to line the inside of the basin, pressing it into the dough. Try to get an even thickness on the sides and bottom.
Next, combine the bacon, onion, sage and brown sugar. Add this mixture into the pudding basin, pressing down to get filling to line up with top of basin.
Next, roll out the remaining dough to 1/8-inch thickness and place on top of the pudding basin. Trim and press edges firmly together, using a bit of cold water.
Finally, cover the basin with parchment and aluminum foil (don't forget the pleat) and tie with string. Steam for 4 hours. Don't forget to check the water level and top it off with more boiling water from the kettle!