Using a fork or pastry blender, 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 and rest for 15 minutes.
Rub a little oil around the inside of a 20cm/8” round baking dish or tart tin, or insert a baking liner and pre-heat the oven.
Dust the work surface with flour, put the dough in the middle and sprinkle it with more flour.
Roll the pastry out into a circle 5cm/2” larger dish and lift it into the dish OR simply press the pastry into the baking dish using your fingers.
Lay a large piece of parchment over the raw pastry, making sure all the edges are covered.
Scatter the ceramic baking beans (OR rice OR flour) over the parchment, spreading them all over the surface.
Bake for 10-12 minutes.
Remove from the oven and carefully lift the parchment and contents off the pastry.
Return the pastry to the oven and cook for a further 4-5 minutes without browning too much.
Put the oil into a frying pan over medium heat.
Slice the mushrooms, add them to the pan, cook until they start changing colour, then remove from the heat.
Break the eggs into a bowl and whisk them well.
Add the mushrooms, season with salt and pepper and mix well.
Pour the mixture into the pastry case.
Bake for 35-40 minutes.
20cm/8” round baking dish or tart tin, frying pan and mixing bowl
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!