Slice the chicken into thin strips. Pour over 2 tbsp of the soy and marinate in the fridge, covered, for 1 hr.
Meanwhile, heat 1 tbsp of the oil in a wok or deep frying pan over a medium heat and cook the peppers for 3-4 mins until just beginning to soften. Remove with a slotted spoon and set aside.
Spread the cornflour out in a shallow bowl. Season, then add the marinated chicken strips and toss to coat well.
Fill a wok or deep frying pan with the oil so it’s about ½cm deep, then heat to medium-high. Carefully add the chicken in batches, cooking for 3-4 mins, using tongs to turn regularly until golden and crispy. Remove with a slotted spoon and place on a plate lined with kitchen paper to drain.
Mix together the remaining soy, sweet chilli, garlic paste and sesame oil in a large bowl. Add the cooked peppers, crispy chicken and toss together until coated all over and sticky. Serve sprinkled with the spring onions.