Heat the oil in a large pot and fry the onion until soft. Add the carrots, celery and potato. Let sweat over a gentle heat for about 5 minutes. Add the rest of the ingredients, bring to the boil and then simmer for about 20 minutes.
Meanwhile, prepare the dumplings. Rub the butter into the flour until it resembles breadcrumbs. Add the herb, salt and pepper and mix with the egg, adding enough water to form a sticky dough.
Using a wet spoon drop tablespoons of the dough on top of the simmering stew. Place the cover on the pot and leave for about 10 minutes.Remove a dumpling, if when you cut into it its fluffy and "bready", it's done!! Remove them all at the one time, just like bread, they'll get soggy if you leave them too long.