For the duck, rub salt into the legs and place on a stainless steel rack with another tray underneath to catch drips. Refrigerate for 3 hours.
Preheat an oven to 120°C.
Wipe salt from duck legs with a cloth. Place legs and all remaining ingredients except the duck fat in a deep roasting tray.
Heat duck fat in a saucepan. When simmering, pour over legs. Cover with foil, place in oven and cook for 3-4 hours, until the duck meat is tender. Ensure the duck fat doesn’t get too hot otherwise the legs will fry and become dry and stringy.
For the relish, mix sliced onion with salt; allow to soften then squeeze liquid off. Wrap pepper, allspice, chilli, cinnamon and clove into muslin cloth.
Combine vinegars, sugar, spice bag and onion in a small saucepan. Simmer until onion turns translucent. Add julienned beetroot and slowly cook until beetroot is very bright and starting to soften slightly, about 5-10 minutes. Remove spice bag. Finish with vino cotto.
Remove duck legs from oven. Allow to cool in fat for at least half an hour.
Lift out legs with a slotted spoon and drain. Serve duck with relish, silverbeet and freshly ground pepper.