Preheat the oven to 150 degrees C. Put the cubes of beef, flour and salt and freshly ground black pepper in a plastic bag. Seal the bag tightly, then shake vigorously so the meat is coated in the flour. (Alternatively, if you don't have a plastic bag, you can toss the beef and flour together with your hands in a large bowl.)
Heat 1 tablespoon of the oil in a large heavy-based frying pan over a high heat. Add the meat in batches and cook, turning frequently, until browned. Once each batch is browned use a slotted spoon to lift the meat into a large flameproof casserole dish. Add the bay leaves to the browned meat in the dish.
Add the remaining oil to the frying pan and gently cook the onion, stirring frequently, for 10 minutes until soft and translucent. Add the garlic, root vegetables and celery and cook for a further 2–3 minutes, or until just starting to colour. Lift all the vegetables into the casserole dish with a slotted spoon.
Deglaze the frying pan by pouring in a little of the stock. Bring to the boil, scraping up any crusty sediment from the bottom of the pan, then stir in the tomato paste, vinegar, sugar and dried mixed herbs. When blended, pour the mixture over the vegetables. Pour the remaining stock into the casserole dish.
Bring the casserole to the boil, then cover tightly with a lid and transfer to the preheated oven. Cook for 2 hours, or until the beef and vegetables are tender.
Meanwhile, to make the dumplings, sift the flour and the mustard, if you are using it, into a bowl. Add a pinch of salt. Stir in the suet, shortening or butter and rosemary, then add about 100 ml of cold water to make a soft, but not sticky dough. Divide into twelve equal pieces and lightly roll into balls with floured hands.
Remove the casserole dish from the oven after 2 hours and increase the oven temperature to 160 degrees C. Arrange the dumplings on top, spreading them out evenly. (It is important that the liquid is hot and bubbling when the dumplings are added to the casserole, to ensure that they rise well.) Cover again and return to the oven for a further 20 minutes, or until the dumplings are well risen, light and fluffy. Serve straight away on warmed plates. Garnish with chopped parsley.