Put the duck breasts, skin side down, in a cold ovenproof dish and cook over a medium heat for 6 minutes or until the skin has turned crisp and brown. Turn them over and cook for a further 2 minutes. Transfer the duck to a preheated oven, 180°C (350°F), Gas Mark 4, and cook for 5 minutes until cooked through. Remove the duck breasts from the oven, cover with foil and leave to rest.
Meanwhile, blanch the green beans in lightly salted boiling water for 2 minutes until cooked but still firm and bright green. Drain and refresh in cold water. Transfer the beans to a large salad bowl with the clementine segments.
Make the dressing by whisking together the clementine juice, vinegar and oil in a small bowl. Season to taste with salt and pepper.
Add the tatsoi or spinach to the beans and clementines, drizzle over the dressing and combine well. Slice the duck meat, combine it with the salad and serve immediately.