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Heat the slow cooker if necessary. Fry the onion in the oil in a frying pan with a splash of water for 10 minutes or until it is soft but not coloured. Add the mushroom slices and stir them around until they start to soften and release their juices.
Meanwhile pour the stock into a saucepan and add the porcini, bring to a simmer and then leave to soak. Tip the onions and mushrooms into the slow cooker and add the rice, stir it in well. Pour over the stock and porcini leaving any bits of sediment in the saucepan (or pour the mixture through a fine sieve).
Cook on High for 3 hours, stirring halfway. and then check the consistency – the rice should be cooked. If it needs a little more liquid stir in a splash of stock. Stir in the parsley and season. Serve with the parmesan.