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What you need for mushroom ketchup
400g fresh mushrooms of different types
1tbsp sea salt flakes
20g dried porcini mushrooms
1 onion, finely chopped
Pinch of bicarbonate of soda
3 cloves garlic, finely chopped
Pinch of allspice
Pinch of nutmeg
Black pepper
100ml cider vinegar
2tbsp brown sugar
2tbsp olive oil

ALTHOUGH mushrooms come in many shapes and sizes, the most commonly eaten types in Europe are one species, Agaricus bisporus. We know them as button mushrooms and chestnut mushrooms when they are young and portobello mushrooms when they are fully grown.

Most of the organism is made up of an underground network of thin fibres called hyphae. The part we eat is the fruiting body, which emerges above the soil and releases spores into the air. The more highly prized types of mushroom, such as chanterelles, morels and porcini, grow in symbiosis with trees, so they are harder to cultivate and are usually harvested from the wild. Many mushrooms produce poisons to protect themselves from being eaten by animals. As such, foraging for them isn’t recommended without expert knowledge.

Mushrooms are 80 to 90 per cent water. Their cell walls contain chitin, a quite different polymer to the cellulose of plant cell walls, which accounts for their strange texture. Chitin is remarkably heat-stable, which is why it is practically impossible to overcook mushrooms – they remain firm yet tender even after prolonged cooking.

They are also higher in protein than most plant foods and contain more glutamate, the chemical responsible for the savoury umami taste. The distinctive aroma of mushrooms comes partly from octenol, an alcohol molecule with eight carbon atoms. It is made by enzymes when the tissue is damaged, which happens most in the gills (the ribs beneath the cap). This is why closed cup mushrooms are less flavoursome than types such as portobello.

Although ketchup is today associated with tomatoes, it first became popular in Britain and the US as a mushroom-based sauce in the 18th century. In historical recipes, the mixture is strained to produce a thin liquid akin to soy sauce that packs a similar umami punch. I prefer to puree the mixture, making a thick sauce more like the ketchup we are familiar with.

Slice the fresh mushrooms or break them up by hand. In a large bowl, mix them with the sea salt, squeezing and bruising them with your hands. Cover and leave for 24 hours. Soak the dried porcini mushrooms in 200 millilitres of hot water for 30 minutes. Sauté the onion with a pinch of bicarbonate of soda so it softens and browns more quickly.

Once starting to brown, add the garlic and cook for 2 minutes. Then add the fresh and rehydrated mushrooms along with any liquid in the bowls, being careful to leave any grit behind. Add the spices and simmer for 45 minutes, adding water if it starts to become dry.

Blend the mixture with the vinegar, sugar and olive oil until smooth. Taste and add more salt, sugar or vinegar as needed.

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