Put the flour, salt and suet in the mixing bowl and stir to mix.
Pour in most of the water and use a knife to stir together. When the dough starts to come together use floured hands to bring it together in a bowl. Use the remaining water if it seems too dry.
Lightly flour the work surface and rolling pin.
Divide the dough into a third for the lid and the remaining two thirds for the basin.
Lightly grease the basin with oil or butter, then roll out the pastry. Line the basin with the pastry and let the excess hang over the edge.
Add the seasoning to the flour and use it to coat the beef and kidneys. Mix in the onion and transfer to the lined basin.
Pour in the beef stock.
Roll out the pastry for the lid and place on top, pressing to seal. Trim off the excess pastry.
Cut 2 pieces of parchment or foil with sufficient to cover the top and halfway down the outside. Fold a pleat in the middle, which will allow the steak and kidney pudding to puff up as it cooks. Lightly grease the underside and then tie the parchment on with string. Tie more string around the basin like a parcel, leaving enough slack to remove the pudding safely when it's cooked.
Preheat the slow cooker on the HIGH setting.
Place the pudding inside and pour boiling water to come halfway up the basin.
Cook on high for 6-8 hours on HIGH.
Remove the pudding carefully and leave to rest for 10 minutes. Take off the paper and run a knife around the top edge to loosen.
Turn the pudding out onto a dinner plate to serve.