Take two or four steaks of frying or braising steak. Dust them in plain flour, salt and pepper and fry them for a few minutes, either side, in a hot pan and a little oil. Transfer them to the slow cooker. Fry one large sliced onion in a little more oil in the same pan as the steaks until translucent or starting to brown. Put them in the slow cooker.
Add enough beef stock to just cover the meat and cook on high for one hour then on low for about two hours, or until the beef is tender. When starting use a bit of the stock to deglaze the frying pan and add it to the SC. After cooking carefully remove the steaks and pour the fluid into a saucepan, put the steaks back in the SC.
Mix one teaspoon of flour/cornflour with 1/2 teaspoon of mustard powder and a little water and add to the saucepan. Heat until thickened then add to the steaks in the SC. Cook everything on low for a further 1/2 hour and serve.