Soak the butter beans in cold water for 24 hours.
Combine flour, baking powder, onion, oil, and seasoning in a bowl for the dumpling.
Mix in enough water to form a soft dough and shape into an oval.
Wrap the dough in clingfilm and chill in the fridge for 30 minutes.
Preheat the oven to 120°C.
Wash and drain the barley and drain the soaked butter beans.
Grease a large heavy casserole dish with goose fat or sunflower oil.
Place the cubed beef in the center of the dish.
Arrange the dumpling, beans, barley, and potatoes around the meat.
Pour in boiling water to cover ingredients and season with salt and pepper.
Cover with a tight lid and cook on a low oven shelf overnight.
Check liquid levels in the morning and add more water if necessary before serving at lunch.