Preheat oven to 300ºF/150ºC
In a dry frying pan, lightly toast the sesame and cumin seeds. Leave to cool slightly then grind into a semi fine powder in a spice grinder or pestle & mortar. Stir in the sumac, cinnamon, thyme, salt and pepper. Set aside.
Wash and pat dry the duck with paper towels inside and out. Place the duck into a roasting tin. Drizzle over the oil and rub into the bird then sprinkle over all the za'atar to cover. Arrange two of the lemons around and underneath the duck.
Cover the roasting tin with foil and roast for 3 ½ hours - baste the duck with the cooking juices every 30 minutes or so. Remove the foil and turn up the heat to 180ºC to crispen up the skin.
Remove from the oven and rest for 5-10 minutes before gently removing as much meat as you can with two forks to a waiting serving platter.
Serve with the remaining lemon wedges to squeeze over the duck, alongside salad, pilaf or vegetables.