Preheat the oven to 160c/gas3. Heat the oil in a large frying pan (preferably non stick). Seal the chicken on both sides - it will turn white. Place in casserole dish.
Add the megez, pepper and garlic to the pan and cook for a few minutes, keep stirring.
Add the tomatoes and the stock then bring to the boil.
Pour the tomato mixture over the chicken, cover with a tight fitting lid and place in the oven to cook for 1 hour.
Remove the casserole and take out the chicken. Blend the cornflour with 1 tbsp cold water to form paste, add 2tbsp of the gravy to the cornflour and stir into the casserole. Heat gently on the stove top, stirring well until slightly thickened or return to the oven for a further 5 minutes.
Return the chicken to the casserole dish and add the olives. Remove rosemary sprigs before serving.