Slice the leeks and wash them properly in a bowl of water — grit hides between the layers and will ruin the soup.

Melt the butter and sweat the leeks with a pinch of salt over low heat for 10 minutes, covered. Do not let them colour; browning makes the soup grey.

Add the peeled, diced potatoes, thyme and broth. Simmer 20 minutes until the potato falls apart under a spoon.

Blend until just smooth. Stop as soon as it is — over-blending potato turns it gluey and there is no way back.

Season generously and loosen with a little hot broth if it thickens too much.