The day before cooking, put the cumin and the coriander seeds in a dry pan and toast until they are slightly coloured or aromatic. Remove to a board and crush them with the flat side of a knife blade, add to a pestle and mortar and grind along with the juniper berries, sliced garlic clove and salt. Rub the salt mixture all over the duck legs.
Arrange the duck legs in an ovenproof dish and sprinkle over the thyme and rosemary leaves. Place the halved garlic head on top of the duck legs. Set aside, covered, in a cool place for at least four hours, turning them over two or three times as they cure.
After four hours, suspend a rack over a roasting tin. Transfer the confit duck legs to the rack and chill in the fridge for at least six hours, or overnight. After six hours, turn the duck legs over and chill for a further two hours.
When the duck legs have cured fully, shake off the salt mixture and pat the duck legs dry using kitchen paper. (NB: Do not rinse.)
Place half of the duck or goose fat into a heavy-based, cast-iron pan. Press half of the confit duck legs into the fat, covering the duck legs. Repeat the process with the remaining duck legs and duck or goose fat, so that the duck legs are layered in the fat. Sprinkle over the bay leaves and peppercorns.
Preheat the oven to 150C/300F/Gas 3.
When cooking the confit duck, increase the oven temperature in increments of 10C every 15 minutes to a peak of 180C/350F/Gas 4 for the first 45 minutes of cooking, then reduce the temperature steadily to 150C/300F/Gas 3 every 15 minutes for the next 45 minutes until the cooking time is over. This helps to keep the meat moist. Alternatively, cook the meat at a constant 160C/320F/Gas 3 for 1 hour 30 minutes. When the cooking time is over, the meat should almost be falling off the bone. (NB: At this point, the confit duck can be cooled in the fat and stored, covered in the fat, in an airtight container in the fridge. It will keep for weeks if properly covered in the fat.)
Otherwise, for the parsnip purée, place the parsnip, potato and apple chunks into a large saucepan. Add a pinch of salt and white pepper.
Pour over the olive oil and milk, then add enough water to just cover the vegetables. Carefully bring the mixture to the boil, then reduce the heat and simmer for 12-15 minutes, or until the vegetables and apple are tender.
Drain the liquid from the pan, reserving in a clean bowl. Allow the cooked vegetables to cool a little, then transfer to a food processor and blend to a purée. As you blend the mixture, gradually add enough of the reserved cooking liquid to whip the vegetables into a smooth purée. Season again, to taste, with salt and white pepper. Keep warm.
To serve, spoon two duck confit legs onto each of four serving plates. Spoon a dollop of the parsnip purée alongside and sprinkle the roasted cobnuts over the purée (if using).