Start this recipe 2 days before serving.
You will need at least a 6 litre (24 cup) flameproof casserole dish.
Soak the beans and barley together in a large bowl in plenty of water overnight. The next day, drain and rinse.
Preheat the oven to 130°C (250°F/Gas 2).
Saute the onion in the oil in a frying pan over medium - high heat until golden brown, about 15 minutes. Remove the onion and set aside.
Put the brisket in the bottom of the casserole dish (see NOTE below). Divide each of the onion, beans/barley and potato into three. In that order, put one layer of each, then half the top rib, another layer of each, the remaining top rib, the remaining layer of each, finishing with a layer of potato.
Dissolve the salt in the boiling water and pour in the dish. It should come to just under the top layer of potatoes (you may not need it all OR add more water if necessary). Bring to the boil on the stovetop and then place an open brown paper bag or some baking paper on top of the potato layer and cover with a lid.
Place in the oven and bake for 2 hours.
Turn the oven down to 100°C (200°F) and cook for a further 20 - 22 hours, checking for time to time. Add more water if it looks like it is drying out too much.
NOTE re cooking time: Alternatively, cook at 150°C (300°F/Gas 2) for 4 1/2 hours.
NOTE re brisket: We like to cut the brisket into 3 pieces, across the grain, before placing in the bottom of the casserole dish.