Melt the butter with the oil in a large saucepan over a medium heat, add the leeks and cook for 5 minutes until starting to soften.
Stir the potatoes and apples into the pan, cover and cook for a further 5 minutes.
Add the cider and cook, uncovered, until reduced by half. Stir in the stock, cover and simmer for 15 minutes until the potatoes are tender.
Serve topped with grated Gruyère cheese.