Soak the porcini in about 750ml warm water for 30 minutes. Remove the mushrooms with a slotted spoon, reserving the soaking water. Rinse the mushrooms briefly under the cold tap (they can be gritty) and pat dry with kitchen paper. Roughly chop them.
Strain the mushroom soaking liquid through a sieve lined with muslin or kitchen paper into a bowl.
Heat the butter and olive oil in a saucepan over a medium-low heat and sweat the onion, stirring from time to time, for about 15 minutes, until soft and translucent. Add the garlic and stir for a minute, then add the mushrooms and cook, stirring, for another couple of minutes.
Add the passata or the tomatoes with their juice, chickpeas, rosemary, reserved mushroom soaking liquid and a few grinds of black pepper.
Bring to the boil, turn down the heat and simmer gently for about 45 minutes. Add the shredded chard or spinach and cook for a further 8–10 minutes for chard, just 2–3 minutes for spinach.
If the soup seems too thick, thin it slightly with a little water. Discard the rosemary. Taste and add salt and pepper if needed.
Ladle into warm bowls, trickle over some olive oil and use a vegetable peeler to shave a few slivers of hard cheese over the top, if you like. Serve at once.
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