For the onions: put the onion in a saucepan and add the sugar and vinegar. Simmer until the onions have softened and the syrup has cooked down until thickened and sticky. Season and set aside.
For the mango sauce: heat the oil in a saucepan and fry the diced mango with a pinch of salt for 8-10 minutes, until very soft. Tip the fruit into a liquidiser and blend until smooth. Push through a sieve and stir in the chopped chilli. Leave on one side.
For the chicken: split the chicken breasts in half horizontally, and using a mallet or the base of a heavy pan, bat out each breast between 2 sheets of plastic until flattened – about 0.5cm thickness.
Coat the chicken with the jerk sauce and pepper. Leave to marinate in a covered bowl, inside the fridge for an hour or two – overnight if you have the time.
Preheat a heavy skillet, and cook the chicken on a high heat for about 3 minutes on each side until tender. After cooling the chicken for a minute, cut the meat into thin shreds.
Fill a large bowl with about 0.5-litre lukewarm water; stir in the vinegar and add salt and the sugar.
Briefly dip each rice paper wrapper into the warm water. As soon as it softens, lay it out on a damp tea towel, dabbing away any excess water with a paper towel.
In the centre of each wrapper put a few salad leaves and 2-3 mint leaves. Pile on a generous teaspoon each of shredded carrots, spring onions and cucumber. Top with a heap of shredded chicken. Finish with a sprinkling of pickled red onions and a sprinkling of sesame seeds.
Fold over the ends of each wrapper and roll around the filling in a cigar shape. Halve each roll for easy handling, and serve with the mango dipping sauce.