Dust the steak and kidney pieces with the seasoned flour, then shake off any excess. Heat a dash of vegetable oil in a large, heavy frying pan and fry the meat and kidneys in small batches for 3-4 minutes, or until browned all over, adding more oil if necessary with each batch. Transfer the cooked pieces to a clean pan.
Deglaze the pan the meat and kidneys were cooked in with the red wine, scraping off all the residue from the pan with a wooden spoon, then pour over the beef and kidneys.
Wipe the pan with kitchen paper, then add a dash of oil and fry the chopped onion for 2-3 minutes, or until softened but not coloured. Add the onion to the pan with the meat.
Add the mushrooms to the frying pan and fry for 1-2 minutes, then pile into the saucepan along with the bay leaf, Worcestershire sauce and enough stock to barely cover the meat. Season with salt and freshly ground black pepper, then stir and bring to a simmer.
Cut a piece of greaseproof paper into a circle the same size as the pan. Dampen the paper, then cover the stew with the dampened greaseproof paper, put on the lid and simmer for one hour and 30 minutes, or until the meat is just tender. Remove from the heat and leave to cool.
For the crust, mix the flour, baking powder, suet, breadcrumbs and salt together in a bowl and gradually mix in enough water to form a soft dough. (You may not need all of the water.)
Roll out three quarters of the dough to a circle 5mm/¼in thick and slightly larger than a 1.2 litre/2 pint pudding basin. Butter the basin and press in the dough, making sure there is some overhang from the edges.
Spoon the steak and kidney into the pudding basin, then pour in enough gravy to come just below the top of the meat.
Roll out the remaining dough into a circle 5mm/¼in thick, moisten the edges and place over the top of the basin. Pinch the edges firmly together and trim off any excess.
Cut out a circle of greaseproof paper or foil about 5cm/2in larger than the top of the pudding, grease one side and lay it greased side down over the pastry. Tie the paper to the pudding with string, making a handle over the top to help remove it from the pan later. Do not pull the cover tight over the top - leave a little to balloon up as the pudding expands while cooking.
Steam in a covered steamer or saucepan half full of simmering water for one hour and 30 minutes, or until the pastry is cooked through.
For the onion gravy, heat the oil in a frying pan and add the onion. Lightly fry for 15 minutes, or until the onion is golden-brown but not burnt. Pour in the wine and stock, reduce the heat and allow to simmer for 10-15 minutes, or until the liquid has reduced by half. Stir in the mustard and season, to taste, with salt and freshly ground black pepper.
Serve the pudding in wedges with the onion gravy spooned on the side.