Mix dry ingredients in food processor briefly.
Add butter and mix until crumbly. While the food processor is running, add egg and water until just combined. Remove to counter and knead until it comes together.
To Make the Quiche:
Cook the mushrooms: in a pan over medium heat, melt the butter. Just before the butter beings to brown add the mushrooms, season with salt and pepper and cook until all moisture has been released and evaporated. Set aside.
Roll out the pastry dough into a removeable bottom tart pan. Bake at 350°F for 22 minutes or until light golden brown. Be sure to use pie weights for the first 10 minutes to ensure the pie crust doesn’t develop air pockets. Remove from the oven and cool to room temperature. The crust can be cooked a day in advance. Keeping the shell in its pan, place on a baking sheet and spread the mushrooms and caramelized onions out evenly on the crust and sprinkle on the cheese. Slowly add the custard until it reaches the top of the crust but doesn’t spill over.
Bake in a 350°F oven for 35 minutes, until the custard is completely set.
Remove from the oven. Allow to cool 5 minutes, then carefully separate crust from the tart pan. Allow to cool to room temperature. *Removing the pan ensures the quiche sides will stay crisp and not get soggy.