Toss the beef in the flour. Fry the onions in the oil for 10 mins. Add the beef in batches until evenly browned. Add carrots, celery, herbs, stock and stout. Bring to the boil, stirring.
Add sugar, Worcestershire and season. Cover and simmer for 2 hours.
Make the dumplings – mix flour, breadcrumbs, suet, mustard seeds and herbs. Add 5-6 tbsp water and mix to a soft dough. Divide into 8, shape into balls and add to the beef. Cover and cook for 20 mins. Serve with mash.