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To make the pastry, tip the flour and butter into a bowl, then rub together with your fingertips until completely mixed and crumbly. Add 8 tbsp cold water, then bring everything together with your hands until just combined. Roll into a ball and use straight away or chill for up to 2 days. The pastry can also be frozen for up to a month.
Roll out the pastry on a lightly floured surface to a round about 5cm larger than a 25cm tin. Use your rolling pin to lift it up, then drape over the tart case so there is an overhang of pastry on the sides. Using a small ball of pastry scraps, push the pastry into the corners of the tin (see picture, above left). Chill in the fridge or freezer for 20 mins. Heat oven to 200C/fan 180C/gas 6.
While the pastry is chilling, heat the butter in a pan and cook the leeks for 10 mins, stirring occasionally, until they soften. Then turn up the heat and add the mushrooms. Cook for 5 mins more, then turn off the heat.
Lightly prick the base of the tart with a fork, line the tart case with a large circle of greaseproof paper or foil, then fill with baking beans. Blind-bake the tart for 20 mins, remove the paper and beans, then continue to cook for 5-10 mins until biscuit brown
While the tart case cooks, beat the eggs in a bowl, then gradually add the cream. Stir in the leeks, mushrooms and half the cheese. Season, then tip the filling into the tart case. Sprinkle with the rest of the cheese, then bake for 20-25 mins until set and golden brown. Leave to cool in the case, trim the edges of the pastry, then remove and serve in slices.