Put the meat into the freezer while you prepare the other ingredients (this will help when slicing). Combine the stock, soy, mirin and sugar in a pan, bring to the boil, then turn down to a simmer. Add the onion and cook for 5 minutes. Meanwhile, slice the meat as thinly as you can, add the beef to the pan and cook for a few minutes, stirring, until opaque. Serve over steamed rice with a sprinkle of the pickled ginger, spring onion and chilli.