Heat a griddle pan until it is very hot and fry the duck, skin side down, for 4 minutes or until golden brown. Turn the duck over and cook for 2 minutes, then transfer it to a preheated oven, 190°C (375°F), Gas Mark 5, and cook for 6—8 minutes until cooked through. Remove from the oven, cover with foil and leave to rest.
Meanwhile, halve the peaches and remove the stones. Heat a griddle pan to a medium heat, add the peach halves, cut side down, and cook until they are golden yellow. Cut the peaches into wedges and mix them in a bowl with the hazelnuts and rocket.
Make the dressing by whisking together the mustard, vinegar and oil. Season to taste with salt and pepper.
Thinly slice the duck meat and add it to the salad. Drizzle over the dressing, combine gently and serve.