Preheat the oven to 180°C/350°F/Gas Mark 4. Bring a pot of water to boil. Cut the potatoes into thin slices. Add the potatoes to the boiling water and cook for 12 minutes.

Cut the leek into thin slices. Crush the garlic. Grate the cheese.

Heat a frying pan over high heat. Once hot, add the bacon lardons and leek to the frying pan and cook for 6 minutes. Add half the garlic and fry for 1 minute.

Add the crème fraîche, vegetable stock, parmesan, and 50ml of water. Mix together, bring to a boil, then reduce heat and simmer for 2 minutes. Remove from heat and stir in the blue cheese until melted.

In an oven-proof dish, layer the cooked potatoes. Pour the sauce over the potatoes. Cover with puff pastry. Place in the oven and cook for 25 minutes.

When there are 8 minutes left on the pie, add the green beans to the frying pan and fry for 6 minutes. Add the rest of the garlic and fry for another 1 minute.

Cut the pie in half and place on plates. Serve with the green beans on the side.