Preheat the oven to 200ºC, gas mark 6. Bring the stock to a boil in a large pan, add the potatoes and cook for 5 minutes.
Meanwhile, heat the oil in a frying pan and fry the leeks for 5 minutes. Stir in the beans and season.
Place 1/3 of the potatoes in the base of an ovenproof serving dish, top with half the beetroot and then half the leek mixture. Repeat with the remaining ingredients. Finish with a layer of potatoes, pouring over the leftover stock. Bake for 25 minutes until golden and cooked through.