Preheat the oven to 180°C. Wipe the chicken joints with paper towels. Melt the butter with the oil in a large, flameproof casserole, and fry the chicken pieces, four at a time, on medium heat for 10 minutes, or until golden. Transfer to a plate. Add the onions, celery and carrots to the pan and cook on medium heat, stirring frequently, for 10 minutes, or until all the vegetables have softened.
Add the flour to the casserole and stir for 1 minute, then add the tomatoes and stock and bring to the boil, stirring. Add the bay leaves and season to taste. Return the chicken pieces to the casserole, coating them well with the sauce. Cover and cook in the centre of the oven for 1 1/2 hours.
When the casserole has been cooking for 55 minutes, make the dumplings. Sift the flour into a bowl. Stir in the suet, two-thirds of the cheese, the mustard and a pinch of salt. Make a well in the centre, pour in the milk and water and mix to a smooth, soft dough – the dough should not be too sticky.
Increase the oven temperature to 200°C and uncover the casserole. Divide the dumpling mixture into eight equal portions and place them, spaced a little apart, on top of the chicken mixture. Cover and cook for 30 minutes, or until the dumplings have risen and are cooked through.
Uncover the casserole, sprinkle with the remaining cheese and cook for a further 5 minutes, or until lightly browned. Alternatively, if you have a grill element inside your oven, switch it on to brown the top of the dumplings. Sprinkle with chopped parsley and garnish with the fresh celery leaves, if using. Serve with steamed green beans or broccoli.