Start by pickling the carrots; in a small saucepan, heat the vinegar and sugar over medium heat until sugar has dissolved. Set aside to cool.
To julienne the carrot, use a vegetable peeler to shave the carrot into ribbons, then use a knife to cut into matchsticks. Add carrot to the vinegar mixture and chill in fridge for at least 2 hours.
To marinate the duck breast, mix the lemongrass, garlic, Chinese five sauce, char siu sauce, soy sauce, honey and vegetable oil together in a large bowl. Lightly score the skin of the duck breasts and cover in the marinade. Cover and chill for 2 hours or overnight. Preheat oven to 200°C, line a baking tray with non-stick baking paper and set aside.
Place the duck breasts skin side down on a frying pan and place on the stove. Turn heat on medium and cook for 8 minutes to render the fat. Turn and cook for a further minute. Transfer the duck to the prepared baking tray and brush with remaining marinade. Place in the pre-heated oven for 8-10 minutes. Remove from the oven, rest for 5 minutes before slicing.
To assemble the banh mi, split rolls down the centre top. Spread mayonnaise on one side. Add 1-2 slices of cucumber, plenty of pickled carrot and coriander. Top with sliced duck, chillies and chilli sauce.