1.Rinse chicken under cold water; pat dry inside and out with absorbent paper. Using kitchen scissors, cut along both sides of backbone; discard backbone. Press down on breastbone to flatten out chicken. Combine half the combined ginger, cumin and coriander in a small bowl; rub mixture over chicken.
2.Preheat oven to 220°C (200°C fan-forced).
3.Heat oil in tagine or flame-proof casserole dish on stove top; cook chicken until browned all over. Remove from tagine. Reserve 1 tablespoon of pan drippings; discard remainder.
4.Heat reserved pan drippings in same tagine; stir in onion and garlic until soft. Add turmeric and saffron, and remaining ginger, cumin and coriander; cook, stirring, for about 1 minute or until fragrant. Add the water, stock, potatoes and pumpkin; top with chicken. Bring to the boil.
5.Cover tagine, transfer to oven; cook for about 1 hour 15 minutes or until chicken is cooked.
6.Stir olives, preserved lemon and juice into sauce; season to taste with salt and freshly ground black pepper. Serve tagine sprinkled with herbs. Serve with couscous.