Put the potatoes in a pan of cold water and bring to the boil. Cook for about 15 minutes, until tender, then drain. When cooled, cut them into bite-sized chunks.
Thinly slice the leek and chop the garlic.
Heat the oil in a large fraying pan and gently sauté the leek for about 8 minutes.
Add the garlic, chilli (if used) and stir for a minute.
Add the beans, then pour in some water or stock (just enough to cover the beans).
Put a lid on and cook for 5 minutes. Add the mustard and vinegar, turn up the heat and reduce the jus.
Stir in the potatoes to warm through.
Just before serving, add the chopped herbs and season to taste.