To make the pastry, sift the flour, nutmeg and salt into a bowl. Mix in the suet and add sufficient chilled water to make a soft dough. Cover and set aside.
To make the filling, mix the flour, salt, pepper and thyme together in a bowl, add the steak and kidneys and stir well until the meat is evenly coated with the flour mixture, then mix in the onion.
On a lightly floured surface, roll out the dough to a round of about 30 cm in diameter. Cut out a quarter and reserve. Fold the remaining dough loosely and place in a lightly buttered 1-litre pudding basin. Unfold and press into the shape of the basin, sealing the join firmly. Allow the excess dough to overhang the rim of the basin. Spoon the meat into the basin. Mix the stock or water with the Worcestershire sauce and pour over the meat. Roll out the reserved dough to a round the size of the top of the basin and place it on the meat. Lightly brush the overhanging dough with cold water, then fold in and press well to seal the meat inside the pastry.
Cover the pudding with a round of buttered greaseproof paper, folded in a pleat in the centre to allow for expansion. Cover the top of the basin with a sheet of pleated foil and secure with string.
Cook for 3 hours in a steamer or large saucepan filled with boiling water to come halfway up the sides of the basin and fitted with a tight lid. Replenish frequently with more boiling water until the pudding is cooked. To serve, tie a folded napkin around the pudding and serve straight from the basin.