Heat oil and half the margarine in a large frying-pan. Add the onion and celery and fry gently for 1 - 2 minutes. Add the turkey and fry briskly for a further 3-4 minutes, stirring often, to brown on all sides.

Sprinkle the flour into the frying-pan and stir over low heat for 1 - 2 minutes. Remove from the heat and stir in the soup and the chicken stock. Return to heat and bring to the boil, stirring. Season to taste. Lower the heat, add the mustard and simmer for 20 minutes.

Heat the oven to 200C/400F/Gas 6.

Meanwhile, bring a large pan of salted water to the boil and cook the macaroni for 10 minutes. Drain well. Melt the remaining margarine in the rinsed-out pan, add the macaroni and stir it well to coat thoroughly.

Spoon the macaroni over the base of a large ovenproof dish.

Stir the sliced mushrooms into the turkey mixture and spoon over macaroni. Sprinkle with the parsley and breadcrumbs and bake for 20 minutes. Serve hot, straight from the dish, garnished with tomato slices.