Steps

Melt the butter in a small saucepan over low heat. Add leeks and cook gently, stirring often, until soft but not browned ~8 mins

Add diced potatoes, water, and a pinch of salt. Bring to a boil, then reduce to a gentle simmer. Cook uncovered until the potatoes are tender and beginning to fall apart ~30 mins

For a rustic soup, mash with a fork or whisk until smoothish. For a silky soup, purée with an immersion blender.

Stir in milk or cream if using, add salt and pepper to taste, and garnish with chives