Preheat oven to 220°C / Gas Mark 7. Rub lamb loin fillets with olive oil and season with sea salt and freshly ground black pepper. Heat a frying pan over high heat. Add lamb and brown for 1 minute each side. Remove from the pan and rest. Add butter to the frying pan. Add mushrooms and cook for 5 minutes, stirring occasionally. Remove. Add the spinach to the pan with the water. Cook until wilted. Remove, cool and squeeze out the liquid. Place lamb in the centre of each pastry sheet and spread with mustard. Spoon mushrooms and spinach over lamb. Brush pastry edges with beaten egg. Wrap pastry over lamb to encase, seal ends and brush top with beaten egg. Place on a baking paper-lined oven tray. Bake for 15-20 minutes (medium) or until golden.
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