Heat oven to 120C/fan 100C/gas ½. Pull any excess fat from the duck’s cavity and use a roasting fork or skewer to prick the skin of the duck all over – do this lightly as you don’t want to pierce the meat. Rub the duck generously inside and out with flaky sea salt, then lay, breast-side down, on a rack over a roasting tray. Roast the duck for 3 hrs, pricking again every now and then should you happen to pass by. While the duck is cooking, score a line gently around the middle of each apple and set aside (this stops them exploding when they cook).
After 3 hrs, remove duck from the oven and turn the heat to 180C/fan 160C/gas 4. Sit the duck on a plate and carefully pour away most of the fat from the tin. Return the duck to the rack, right way up, and continue to roast for 30 mins. Put the apples in under the duck or around the side and roast for a final 30 mins. By now you should have the crispiest duck and apples on the verge of collapse. Leave the duck to rest for 10 mins, then carve and serve with the apples.