1. To prepare the marinade, combine brown sugar and vinegar in a bowl and stir till sugar is dissolved. Add the fish sauce, water, chilli and garlic and stir till well combined. Place the sirloin steaks in a shallow bowl and top with the marinade until submerged. Set aside in the fridge for 1 hour.
2. To prepare the quick pickled carrots, use a vegetable peeler and run it down the length of the carrot to shave off long ribbons and place in a clean jar. Combine the remaining ingredients together in a measuring jug and stir well to combine. Pour the pickle mixture into the jar with the carrot ribbons, seal and set aside for 1 hour.
3. Once the steak has marinated and the carrots have pickled for an hour, heat a cast iron grill pan over medium high heat. Remove the steaks from the marinade, place in the pan and cook for four minutes. Turn the steaks over and cook for another four minutes, then remove from the pan, wrap in aluminium foil and leave to rest for a 4 minutes (timing is for a medium rare steak). Once rested, thinly slice across the grain.
4. To layer your Banh Mi, begin by combining the Kewpie and Sriracha to make a chilli mayo and spread it on the bottom of a cut Vietnamese roll. Top with thinly sliced sirloin steak and ribbons of cucumber. Add ribbons of pickled carrot, a spring onion, coriander leaves and slices of fresh chilli. Repeat layering with remaining rolls and serve.