Heat oil in a large, flameproof casserole. Season chicken pieces and brown all over for 10min. Remove and set aside. Drain fat from the pan - there should be around 75ml (3fl oz). Pour just 1tbsp oil back in to the pan and discard the rest. Preheat oven to 200ºC (180ºC fan oven) mark 6. Add the onion, garlic, celery, carrots, leeks and potatoes to the pan, and cook, stirring, for 5-10min. Stir in the flour, cook for 2min, then pour in the stock. Season well, then return the browned chicken to the pan. Add the rosemary, squeeze over the juice from the lemon half and pop the squeezed lemon half into the pan. Cover and bring the casserole to the boil.