For the duck, rub the salt into the legs and place on a perforated tray with another tray underneath to catch drips while cooking and refrigerate for 3 hours.
Preheat 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 duck fat is simmering, pour over duck legs. Cover with aluminium foil, place in oven and cook for 2-2½ 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.
Remove legs from oven and allow to cool in fat for at least half an hour. Lift from the fat with a slotted spoon and drain. Remove skin, bone, cartilage and excess fat. Shred meat into bite-sized pieces.
For the salad, cut away skin and pith from the grapefruit. Segment it over a bowl to capture juice. Reserve about ½ tbsp juice for dressing. Roughly tear frisee, shred radicchio and chop hazelnuts.
For the dressing, whisk ingredients with about 1/2 tbsp grapefruit juice. Taste and adjust seasoning.
Combine all ingredients in a bowl and gently toss together. Divide over four plates and serve.