Prep the potatoes: boil the potatoes whole for at least 20 minutes, probably a full 30 until they are tender but not completely cooked (the fork should pierce about halfway through). Set aside to cool.

Make the leek sauce: In a small pan, fry the onions and leeks with the butter on low heat until tender, about 10 to 15 minutes. Add salt and pepper. Turn the heat off then pour in the cream, cheese, and top with nutmeg. Stir until the cheese is melted and set aside to cool.

When the potatoes are cool enough to be handled, the cut them into thin slices and set aside.

Assemble: Once the leek sauce and potatoes have cooled, divide the puff pastry into two equal sized pieces and roll or shape the puff pastry into two identical circles. Take the bottom circle and on a lined baking tray start assembling your layers: potatoes in a round on the bottom (leaving a few cms at the edges), some crushed walnuts, cheese, and then topping with leek mixture. Repeat for 2 or 3 more layers until all the potatoes and cream sauce have been used up.

Top the domed structure of potatoes, cheese, and sauce with the second piece of puff pastry and crimp or seal the edges with a fork.

Lightly prick or pattern the top of the pastry with a knife to allow steam to escape before placing in a 180°C oven for 30-35 minutes until the top has browned lightly.