Cook the potatoes in lightly salted boiling water until tender (or use leftover cooked potatoes). Drain and chop.
Slice the onions thinly. Saute in the oil for a few minutes until lightly browned. Stir in the stock, the parsley and the sage. Then stir in the drained beans. Bring to the boil and simmer for a further 2-3 minutes. Remove a little of the stock from the saucepan and cream it with the miso in a cup before stirring into the saucepan.
Serve immediately with rice or bread.