Preheat the oven to 220C/425F/Gas 7.
Heat the oil in a small ovenproof frying pan and season the chicken with salt and freshly ground black pepper.
Add the chicken and fry for two minutes, then turn over and transfer to the oven to roast for 10-12 minutes, or until completely cooked through. Remove and allow to cool.
When cool enough to handle shred the chicken into small pieces.
To serve, place the cooked chicken, watercress, orange, oil and balsamic vinegar into a large bowl and mix well.