First, make a start on the dumplings. Fry the onion in the butter and mix with all the remaining ingredients. Cover and chill for at least 1hr to firm up.
Preheat the oven to 170°C (fan oven 150°C), gas mark 3.
Cut the steak into large bite-sized pieces. Heat 2 tbsp of the oil in a large casserole, then fry the meat, a few pieces at a time, until they are a deep golden brown. Remove with a slotted spoon and set aside.
Add the remaining oil to the pan and brown the carrots and onions for 3-4 mins until they are just beginning to colour. Stir in the flour, scraping up any crusty bits from the base of the pan.
Pour in the stout along with the stock and conserve. Return all the meat with any juices. Bring to the boil, cover tightly with a lid and simmer in the preheated oven for 2 - 2.25 hrs or until meltingly tender.
Meanwhile, with damp hands, shape the chilled dumpling mixture into 12 small balls and return to the fridge. About 30 mins before the end of cooking time, uncover the beef and taste the juices for seasoning. Drop the dumplings on top and return the casserole to the oven, uncovered, to continue cooking for the remaining cooking time.
To make in advance, up to 2 days ahead of time, cook the beef as above but for 1.5 hrs only. Cool and keep in the fridge or freeze. Make the dumpling mixture and keep, covered, in the fridge. When ready to use, thaw the beef if necessary and bring to the boil on the hob, cover, and return to the oven for 1hr. Make and add the dumplings as above.
The dumplings are optional but they are much lighter than traditional suet dumplings so don't make the dish stodgy.
Alternatively, spread 6 thick slices of baguette with mustard and crushed garlic and pop them on top of the beef for the last 30 mins.