In a pan on medium heat add the butter and let melt. Swirl the pan to prevent burning.

When all the butter is melted on medium heat add in 500 grams of cubed and peeled potatoes, 1 large thinly sliced shallot, and 2 thinly sliced leeks (discard the leaves and just slice the white part). Add salt and give a stir to slightly fry the vegetables.

Make a cartouche from some parchment paper (white disk) and place atop the pan. Turn the heat down low and let the veggies simmer and steam under the cartouche for 10 minutes.

Tip the contents into a blender and blend until smooth. Don't let it run for too long or you will make everything gummy.

Pour back into pan to keep warm and taste for seasonings. Add some black pepper and more salt if needed.

Scoop the soup into bowls and microplane over some cured egg yolk. Garnish with some chopped spring onions.