I first had this at a friend's house and it's since become a staple part of my repertoire of recipes. It's another easy but delicious dish, and goes extremely well with the slow cook morrocan lamb dish on this site (#142446).
Preheat the oven to 170 degrees celsius (325 fahrenheit).
In a frying pan, heat half the oil until medium-hot. Cook the chicken in batches until all sides are lightly browned. Remove and set aside.
Add the remaining olive oil to the pan and cook the onions for 10 minutes or until golden. Add the garlic and sumac and cook for a furthr 2 minutes. Spook half the mixture into a deep baking dish and arrange the chicken pieces on top. Cover with the remaining onion, pour in the chicken stock and cover with foil.
Bake the chicken for about 1 hour. Remove the dish from the oven and allow to rest for 5-10 minutes. Season to taste and serve with couscous or rice.