Mise it out: Gather all your ingredients. Trim, wash, and finely chop the leek (or 2 if you want more leek-y flavor or if your leek was on the smaller side) and get it sweating in some olive oil in a heavy-bottomed pot over a moderate heat.
.While the leek is sweating down (careful to make sure it’s not getting color aka browning, peel the potatoes and cut them into small chunks (the smaller the potato is the quicker is cooks)
Heat some stock (Common sense tip: adding cold stock to the pot will stop the cooking–why would you want to do that?) and add the potatoes and stock to the leeks, stirring well. Allow to cook until potatoes are tender. To test: fish one out with a fork and see how soft it is.
Add a couple handfuls of spinach and allow to wilt, salt to taste and allow to cool (pop in the fridge for a cold soup). Blend till smooth, adjust seasoning, and right before you serve stir a generous glug of buttermilk through. Garnish with chives.