Use a fork or pastry blender to work the butter into the flour until the mixture resembles breadcrumbs. Avoid using your fingers for this.
Stir in enough of the water to bring the pastry easily together into a soft, slightly sticky ball of dough - it will appear a little wet but will absorb the liquid while resting.
Cover the pastry with parchment or cling film and rest it in the fridge for 30 minutes.
Oil or parchment line a 20cm/8” round baking dish or tart tin and pre-heat the oven.
Roll the pastry out between two pieces of cling film or parchment into a 25cm/10” circle, or simply press the dough into the baking dish using your fingers.
Put the mushrooms and oil into a frying pan, fry lightly then leave to cool.
Break the eggs into a bowl and beat them well.
Add the cold mushrooms, season with salt and pepper and mix well.
Pour the mixture into the pastry case.
Bake for 30-35 minutes.
20cm/8” round baking dish or tart tin, frying pan
180°C, Fan 160°C, 350°F, Gas 4
Just made this as looking to use up some mushrooms. Found 150 gms not enough so doubled it to 300 gms. This still didn't fill the flan case (8 inches) It's now in the oven but the pastry at the sides of the flan tin has fallen in to the flan . Think this flan case may need to be blind baked first for ten mins or so. Will try that next time. Hope it tastes better than it looks!