Heat oil and add onion and cook for 4 - 5 mins until soft. Add rice and stir through so all grains are coated and cook for 2 mins. Transfer to slow cooker and pour in enough stock to cover. This could be more or less than the 600ml.
Meanwhile, melt butter in pan and cook sliced mushrooms for about 10 mins or until they have browned and liquid evaporated. Stir mushrooms into rice and cook for 2 - 3 hours on low. Stir mid way. Stir at end of cooking and stir in Parmesan Cheese and leave to stand for 5 minutes. Serve with Parmesan shavings on top.
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