Easiest ever soup

Madrid was submerged in snow last Friday. J and I caught a nice little stomach flu that´s doing the rounds, so we´ve been eating plain boiled rice and chicken and broth for days. But now that we´re almost well again, this is the thing.

The easiest soup in the world, and so good. Also I´m fond of it because I associate it with Pía´s birth-day. My last act as a civilian was to stop by the post office and pick up a parcel of books, all Lindsey Bareham´s.
This recipe appeared in all of them, and as I lolled by the pool, I flagged it. It seemed to good to be true, but it was too hot. And then P was born and that was that, until I found myself one day with leeks and potatoes to hand, a sleeping baby, and half an hour.

All you do is clean and slice leeks and peel and cut potatoes while salted water comes to the boil. The vegetables go in, are covered and left to cook until the potatoes are coming apart. This takes somewhere around twenty minutes, and requires no action on your part.
When it´s done, you taste, and say, darn, this is nothing. I can taste leeks and I can taste potatoes, but I can taste a lot of water, and I´m bored.
This is when you follow the last step: add 10 gr. butter per serving. Move it around, watch it melt, swirl a little, try it again.
You won´t beleive the magic. The little atoms of butter galvanize that big pot of soup. The flavours are brought together into a harmonious whole, the water turns from pale to interesting, and you can taste leeks and potatoes, but mostly you can taste soup, gentle and invigorating.

As a lesson in the enlivening properties of fat it has no parangon. It is the soup I would teach a beginner cook, and a no-fat fanatic. Just a little spoonful per bowl, and it turns potatoes to gold.

The instructions are really and truly as simple as described above. The quantities:

500 gr. leeks (white and palest green only)
500 gr. potatoes
1.2 litres water (that´s a stingy five cups or a generous four)
40 gr. butter