Once the steak is cooked, remove to a board to rest. Meanwhile put the tray on the hob over a medium heat and add the red wine, scraping with a wooden spoon to deglaze. Let it bubble until nearly gone and then add the stock. While the stock comes up to the boil stir the cornflour and mustard powder with a tiny bit of water to make a paste, then stir into the stock. Stir or whisk until thickened, checking for seasoning.