Put the dried porcini in a small bowl with 150 ml of warm water. Leave them to soak for 20 minutes.
Drain the chickpeas over a bowl and keep 50 ml of the liquid from the tins (or the cooking liquid if you soak and cook dried chickpeas)
Lift the porcini out and squeeze them over the bowl. Chop them roughly. Strain 100 ml of the soaking water through kitchen paper and set aside.
Heat the olive oil in the saucepan over a low to medium heat. Add the onion, parsley and a pinch of salt then fry for 5 to 6 minutes, until the onion softens.
Add the carrot and celery and cook until soft.
Stir in the porcini and cook for 2 minutes.
Add the sliced chard to the pan along with the tomato paste in chickpea water. Cook for 6 to 8 minutes, stirring now and then, until wilted. Season lightly with salt.
Add the chickpeas, the vegetable stock and the 100 ml of mushroom water. Bring to the boil, then lower the heat and simmer for 20 minutes.
Taste and adjust the salt.
Serve with some rustic crusty bread.
Notes
500 grams of chard looks like far too much, but it's the right amount once it has wilted.
Salt carefully at each stage. Stock brands vary a lot in saltiness, so taste before adding more.