Cut the potatoes into bite-sized chunks then put them in a pot of lightly salted water. Add the bay leaf and bring to the boil. Turn the heat down and simmer for 10 minutes. Drain and set aside (you can cool them or refrigerate for later use).
Thinly slice the the leek.
Heat the oil in a large frying pan and gently sauté the leek for about 8 minutes over low heat.
Add the garlic, oregano and mustard, then cook for 30 seconds. Add the stock and cook until you get a creamy sauce.
Add the boiled potatoes and stir until warmed through.
Serve with fresh herbs scattered on top.