Heat the oil in a heavy-based saucepan and soften the onion. Mince 3 cloves of garlic, add this to the onion together with the grated ginger and turmeric powder. Stir for a minute, then pour in the lemon juice and stock. Add the chicken pieces, the cinnamon stick and saffron, if used. Season with salt & pepper, then bring to a boil and simmer for 30 minutes.
Meanwhile, make the Moroccan gremolata: in a small bowl mix the mint leaves, grated lemon zest, parsley and remaining minced garlic. Set aside.
Transfer the chicken pieces to a plate, reduce the sauce by half - this should take about 10 minutes. Stir in the gremolata and olives, put back the chicken pieces, coat them well in the sauce and warm through.
Serve warm.