Peel and finely slice the onion. Wash the pepper and slice 1cm thick.
Cut away any excess fat from the steak, sprinkle with salt and pepper on both sides and place straight into a hot frying pan and leave for 2 minutes. Turn over and cook for a further 2 minutes for a medium steak. Leave a bit longer for a well done steak if preferred. Remove from the pan to rest.
Turn down the heat to medium and add in the onions, sauté for 1 minute the add in the pepper for a further 1 minute. Then take off the heat and add the soy sauce, Worcester sauce and flour, mix together then return to the heat stirring until the liquid thickens then turn off the heat.
Slice the steak into thin strips and place into the bun, add the onions and peppers onto the steak with squeeze of yellow mustard. Close and eat.