Leek and potato soup, smooth or chunky

Leek and potato soup is the soup I make when the fridge is nearly empty and the shopping has not happened yet. A couple of leeks, a few potatoes, an onion, stock and a bit of butter: that is all. It is gentle, filling and cheap, and everyone in the family eats it, from the smallest to the grown-ups. Some days I blend it smooth and silky, some days I leave it chunky, depending on the mood.

For four bowls I use three medium leeks, about 500 g of floury potatoes such as Maris Piper, one onion, a knob of butter, one litre of vegetable or chicken stock, and a little milk or cream at the end. A bay leaf and a sprig of thyme are nice if I have them.

Leeks hide grit between their layers, so they need careful washing. I trim off the root and the dark, tough tops, then slice the leek in half lengthways and rinse it under running water, fanning the layers apart with my fingers. Then I slice it into half-moons. The darker green parts can go in too if they are tender; they make the soup a bit greener.

I peel the potatoes and cut them into chunks about the size of a walnut. Peeled potatoes waiting on the board can sit in cold water so they do not turn grey. Any potatoes with green patches or long sprouts I either cut very generously or do not use; green potato skin contains a natural toxin, and it is not worth it.

In a large pan I melt the butter over a low heat and add the leeks and the chopped onion with a pinch of salt. I let them sweat gently with the lid on for about ten minutes, stirring now and then, until they are soft and sweet. They should not brown; brown leeks make the soup taste bitter.

Then the potatoes go in, with the hot stock and the herbs. I bring it to the boil, turn it down, and simmer for 15 to 20 minutes, until the potatoes fall apart when pressed with a spoon. I fish out the bay leaf and the thyme stalk.

For a smooth soup I blend it with a stick blender, keeping the blade under the surface, until it is silky. I do not blend it for too long, because potatoes go gluey when overworked. For a chunky soup I just mash some of the potatoes against the side of the pan with a fork, so the soup thickens but there are still soft pieces. Either way, I stir in a splash of milk or cream, taste, and add salt and plenty of black pepper.

We eat it with crusty bread, or with a little grated cheese on top, or with snipped chives.

Potato soup needs the same care as any other soup when it comes to leftovers. I divide it into shallow containers so it cools quickly and put it in the fridge within two hours. It keeps for two days. I reheat it in a pan, stirring, until it is steaming hot all the way through, and only once. It freezes reasonably well, though potato soup can turn a little grainy; a quick whisk or blend after reheating brings it back.

For a baby's portion, I take some out before adding salt and use water or unsalted stock.

Leek and potato soup. Serves 4. About 40 minutes. You need 3 leeks, 500 g floury potatoes, 1 onion, a knob of butter, 1 litre stock, a splash of milk or cream, and a bay leaf and thyme if you like. Sweat the leeks and onion in butter for 10 minutes without browning. Add the potatoes, stock and herbs and simmer for 15–20 minutes. Blend or mash, add the milk and season.

Smooth or chunky in your house?