In a heavy casserole heat 2 tablespoons of olive oil. Add the celery, carrot, onion, mushrooms and bacon and sweat for 2 minutes. Add the tomato purée and cook for 1 minute.Stir in the quartered tomatoes, then the bay, garlic, mace and pinch sugar. Sprinkle over the flour and cook for 1 minute.
Add the sherry and most of the chicken stock (leave a little amount for cooking the sliced mushrooms later). Bring the mixture to the boil and simmer for 20 minutes.Meanwhile, in a frying pan, sauté the sliced mushrooms in 2 tablespoons of olive oil. Add some salt and pepper and the remaining chicken stock, then stir in the cooked chicken.
When the sauce has been cooking for 20 minutes, stir the chicken mixture into the casserole and heat through for 5 minutes. Finish with the chopped parsley and serve with crusty bread.