The soaking water from dried mushrooms goes into the soup

When I open a bag of dried mushrooms, the kitchen fills with the smell of an autumn wood — so strong I want to stand over it with my eyes closed. A handful of them turns a winter soup into something else entirely. But for a long time I made one mistake: I soaked them and poured the brown water down the sink. And that’s where the flavour was.

Now I soak them in a bowl of hot water for about half an hour, until they’re soft. Then I lift them out, squeeze them gently over the bowl and rinse them under the tap, because grit can hide in the folds. I leave the water alone for a couple of minutes so the bits settle, then pour it carefully into the pan through a sieve lined with kitchen paper. The last spoonful, with the sediment, gets tipped away.

The mushrooms get chopped and fried with onion, and the strained water goes into the soup in place of some of the water or stock — with pearl barley, with potatoes, or in a creamy mushroom soup. It’s good for loosening a sauce or a risotto too. If I’ve soaked more than I need, the mushrooms and their water go into the fridge, covered, and get used the next day.

Which mushrooms? Most of the time I use dried porcini, sometimes a mixed bag of wild mushrooms. They are expensive by weight but go a long way: 20 to 30 g is enough to flavour a big pan of soup for four. I buy them from a shop, in a sealed packet, and check the label for any instructions; I never use dried mushrooms of unknown origin or pick wild ones myself without being certain what they are.

The soaking water must always be strained, and not just for neatness. Dried mushrooms often carry fine sand and grit, which sink to the bottom of the bowl. That is why I let the water settle, pour it slowly, and leave the last cloudy spoonful behind. A coffee filter works well too, if you have one.

Soaked mushrooms always need to be cooked, not eaten as they are. I fry them gently with the onion for a few minutes, then they go into the soup and simmer with everything else, so they are properly hot all the way through.

Here is the soup we make most often. For four people: 25 g dried mushrooms soaked in 500 ml of hot water, one onion and one carrot, chopped, a tablespoon of oil or butter, 100 g of pearl barley, rinsed, two or three potatoes, cubed, and about a litre of water or vegetable stock. I fry the onion, carrot and chopped mushrooms in the oil, add the barley, the strained mushroom water and the stock, and simmer for about forty minutes. The potatoes go in for the last twenty, until everything is tender. Salt, pepper, a bay leaf, and a spoonful of soured cream in each bowl.

The soup thickens as it stands, because the barley keeps drinking. If there is some left, I cool it, put it in the fridge, covered, within two hours, and eat it within two days, adding a splash of water when I reheat it until it is steaming hot.

I keep the unopened packet of dried mushrooms in a cool, dry cupboard, and once opened I fold the bag tightly or tip them into a jar with a lid, so they don't absorb damp.

A handful of dried mushrooms and a glass of dark, fragrant water, and an ordinary pan of soup smells as though it has been simmering all day on a farmhouse range. That smell doesn’t go down my sink any more.