Heat half the oil in a wok over a high heat until the oil starts to shimmer. Toss in half the duck, season with salt and pepper and stir-fry for 2 minutes, until browned yet still slightly pink in the centre. Remove the duck using a slotted spoon and wipe the wok clean with kitchen paper. Heat the remaining oil and stir-fry the rest of the duck in the same way.
Toss the cooked duck in a large bowl with the dressing ingredients. Leave to marinate while you prepare the rest of the salad.
Use a vegetable peeler to slice the carrot finely lengthways into paper-thin ribbons. Cut the cucumber in half lengthways and scoop out the seeds using a spoon. Place cut-side down on a chopping board and finely slice on the diagonal. Place the carrot and cucumber in a large bowl, then add the lettuce, celery, spring onions and mint.
Allow the duck to completely cool in the dressing, then toss it into the prepared salad.