Blend half the blackberries in a mini food processor and press through a sieve, discarding the seeds. In a small bowl, add the blackberry purée along with the oil, vinegar and honey. Season well and whisk until well combined. Cover and leave in the fridge until needed.
Rinse the quinoa and add to a medium saucepan. Cover with 700ml freshly boiled water and bring back to the boil. Cover and simmer for 15-18 minutes or until all the water has been absorbed and the quinoa is cooked through. Spread out on a baking tray to cool.
Meanwhile, preheat the oven to 200°C/fan 180°C/gas mark 6. Heat a frying pan over a medium heat. Season the duck breasts, then add to the pan and sear on each side for 3 minutes. Transfer to the oven to cook for a further 8 minutes. Remove and leave to rest for 10 minutes.
Toss the cooled quinoa with the watercress, goat’s cheese and remaining blackberries. Slice the duck diagonally and serve with the salad, along with the dressing.