Preheat the oven to gas 6, 200°C, fan 180°C. Cut away 1/3 of the pastry and place the remainder back in the fridge. Lightly dust your worksurface and the roll out the pastry to make a rectangle roughly 15 x 25 cm. Place on a baking sheet and mark all over with a fork. Bake in the oven for 15mins until golden and crisp. Gently press down any puffed bits and leave to cool.
Meanwhile heat the oil in a frying pan. Add the onion and cook for 5 mins until softened. Then add the mushrooms and cook 3 mins more until golden. Leave to cool a little. Then stir together with the rice, breaking up any lumps, salmon, lemon juice and zest and dill.
Arrange the mixture on top of the cooked pastry. Roll out the remaining pastry to make a rectangle approximately 25 x 35cm. Carefully drap this over the mixture and tuck the ends under. Then brush all over with the egg. Bake in the oven for 30 mins until golden.
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