Preheat the oven to 180ºC/350ºF/gas 4. Wash the duck, inside and out, and pat dry with kitchen paper. Place it breast-side up in a roasting tray, and stuff with the orange halves and herbs. Rub a good pinch of salt and pepper inside and outside the duck. Turn the duck breast-side down and roast in the oven for an hour.
After an hour, remove the tray from the oven, carefully spoon out any excess fat, turn the duck over and pop it back in the oven for another hour. After the second hour, remove the duck from the oven and leave it to cool. Strip off all the meat using a fork or your fingers, pile it on a plate and leave to one side.
To make the pie, heat a large casserole pot over a medium heat and add a splash of olive oil. Add the onion, celery and carrots and cook gently for 20 minutes, until softened. Add the shredded duck meat, flour, rhubarb and beef stock, and cook down for 15 minutes until you get a lovely thick stew.
Meanwhile, boil the potatoes and swede together in plenty of salted water until cooked, then drain and mash with the butter and milk. Season well with salt and pepper and put to one side.
When the stew is cooked, transfer it to a deep earthenware baking dish. Grate over half the clove of garlic, a little orange zest and a handful of Parmesan, then give it a good stir. Heat the oven again to 180ºC/350ºF/gas 4 again. Spread the mash over the top, creating little peaks with the back of a spoon, then pop it in the oven for 30 to 35 minutes, until brown and crispy on top.