Preheat the oven to 180C/365F/Gas 4.
Season the chicken breasts with salt and freshly ground black pepper.
Heat an ovenproof frying pan until hot and add the olive oil and the chicken breasts, skin-side down. Cook for 2-3 minutes until golden-brown, then turn the breasts over and cook for a further minute.
Place in the oven for 12-15 minutes, or until completely cooked through. The juices will run clear when pierced with a skewer at the thickest point. Remove from the heat and set aside to rest for a few minutes.
Meanwhile, place a large pan of salted water over a high heat and bring to the boil. Add the beans and leeks and cook for 2-3 minutes, then drain and refresh in iced water.
Place the mustard, vinegar and 250ml/9fl oz olive oil into a bowl and whisk to combine.
Slice the beans and leeks into three on the diagonal and add to the dressing.
Add the shallots, tarragon and parsley and season, to taste, with salt and freshly ground black pepper.
To serve, divide equal amounts of the bean and leek salad among four plates and top each with a roasted chicken breast. Drizzle over any extra dressing remaining in the salad bowl.