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Heat the slow cooker to low. Heat the butter and oil in a frying pan. Add the onion and fry for 5 mins over a low heat until softened and translucent. Add the cavolo nero to the pan and fry for 5 mins or until beginning to wilt. Tip into the slow cooker along with the leeks, stock and 300ml of the cream. Cook with the lid on for 3 hrs, stirring occasionally.
Stir through the remaining cream, the mustard, nutmeg and cheese, as well as some seasoning just before serving. Will keep, covered, in the fridge for up to three days. Reheat in the slow cooker on medium for 45 mins-1 hr.
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