Begin the day before you wish to eat by dry brining the duck. Mix the sumac, sugar and salt in a bowl and rub over the duck flesh, leaving the skin. Turn, skin side up and refrigerate uncovered for 12 hours.
For the cherries, bring everything apart from the cherries to a simmer in a small pan. Pour over the cherries in a sterile jar and leave to pickle for at least 12 hours, although they will keep for 2 weeks in the fridge.
The day you wish to eat, add all the dressing ingredients to a jar and shake to combine.
Preheat the oven to 200C. Pat the duck skin dry and score with a sharp knife, taking care not to penetrate the flesh. Lay, skin side down, in a cold pan and set the heat to medium. Cook for 4-6 minutes, or until the fat has rendered and the skin is crisp.
Transfer the entire pan to the oven and cook for 4-5 minutes for rare. Remove and allow to rest for 15-20 minutes.
To assemble the salad, mix the watercress, cucumber, macadamias and olives in a bowl and drizzle over 2-3 tbsp of dressing. Slice the duck and arrange on top, with some pickled cherries and extra dressing on the side.