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Heat oven to 200C/180C fan/gas 6. Wilt spinach in a colander by pouring a kettleful of hot water over it.
Heat half the oil in a large non-stick pan and fry mushrooms on a high heat until golden. Add garlic and cook for 1 min, then tip in stock, mustard, nutmeg and potatoes. Bubble for a few mins until reduced. Season, then remove from the heat; add crème fraîche and spinach. Pour into a pie dish and allow to cool for a few mins.
Brush filo with remaining oil, quarter sheets then loosely scrunch up and lay on top of pie filling. Bake for 20-25 mins until golden. Serve with vegetables.