Preheat the oven to 425F/220C. Set the racks so that you have enough space in the middle of the oven for your roasting pan with a rack to sit in with the duck on top.
Clean out the duck completely and remove any excess far around the neck etc. Prick the skin all over with a fork (this helps the fat to drain as it cooks and makes the skin more crisp).
Pour the boiling water over the duck, some on each side, with the duck sitting in the rack over a roasting pan. It will cause the skin to change color slightly and tighten up - this too helps it crisp. Pour any water held in the cavity into the pan and pat the duck dry all over. Don't throw away the water in the pan - leave it to steam the duck as it cooks initially.
Rub salt and pepper all over the duck's skin, particularly the breast.
Turn the duck breast side up, then roast in the middle of the oven for 45 minutes (this isn't the full cook, just when you turn).
After the first 45min, put a wooden spoon into the neck space (the handle) and another, the other way round, inside the main cavity and carefully drain out the cavity into the baking pan. Turn the duck over to be back up then put back in the oven.
As the duck is cooking, peel and cut the potatoes and carrots into large chunks. After the duck has cooked around 20-30 minutes on the second side, carefully add the potatoes and carrots into the fat drippings below the rack. Spoon a little of the fat over the top of the vegetables then return the pan to the oven.
Once the duck has been in for 45 minutes on the second side (so 1 ½ hours cooking), remove from the oven and turn again back to breast side up. Return to the oven for a final 45 minutes (giving 2 ¼ hours total).
Peel the parsnip and cut into large chunks. Around 30 minutes before the end of cooking (so once it has been in for 15 mins after the last turn), carefully turn the potatoes and carrots in the bottom of the pan and add the parsnips. Return to the oven for the remaining 30 minutes, then take out of the oven. Remove the vegetables and keep warm while the duck rests around 5-10 minutes before you carve it.
Remove the vegetables, draining them as you do, and keep warm. Drain any liquid from the inside of the duck and transfer it to a cutting board to rest around 5-10 minutes before you carve it.