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Heat the butter in a small frying pan set over a medium heat. Fry the leek for 10 mins until softened. Transfer to a bowl and leave to cool for 15 mins. Fry the bacon in the pan for 5 mins, or until crisp. Mix the leeks with the mustard, parsley, cheddar and mozzarella. Spread the mixture over 1 tortilla and top with the bacon and second tortilla.
Heat a frying pan over a high heat. When hot, add the quesadilla and cook for 2 mins on one side. Turn it over and cook for 1-2 mins more, or until the cheese is melted. Cut into four.