Add cold diced butter and use a pastry cutter, fork, or your hands to mix everything together into a sand-like consistency. If using your hands, pinch the butter into the flour.
Add water 1 tablespoon at a time until the dough just comes together. Use your fingers to gently mix the dough as you add the water. You should be able to take a handful of dough and clump it together without it falling apart.
Gather the dough into a big ball. Cover and chill in the fridge for 1-2 hours.
Quiche
Preheat oven to 375°F/190°C.
Roll out the pie dough and press it into a 9-inch pie pan. Chill in the fridge for 30 minutes (if you're in a rush, you can place it in the freezer for 5-10 minutes, but don't leave it too long or the dough will get too cold and crack during baking).
Par-bake crust: Remove the pie pan from the fridge/freezer. Place a sheet of parchment paper on top and add pie weights or dried beans**1. Bake for 15 minutes, then remove the pie weights and parchment paper and bake for another 5 minutes or until golden on the edges. Set the crust aside while you prepare the filling. Reduce oven temperature to 350°F/180°C.
Sauté veggies: Heat olive oil in a skillet over medium heat. Add leeks and a pinch of salt and cook until leeks are soft, about 5 minutes. Add garlic, mushrooms, and zucchini and cook another 3 minutes or until veggies are tender. Remove from heat and drain off any excess water left in the pan. Set aside while you prepare the tofu filling.
Blend tofu filling: To a blender or food processor, add tofu, soy milk, nutritional yeast, paprika, herbs, salt, pepper, and turmeric. Blend until smooth. Pour into the pan with the veggies and mix to distribute.
Fill pie crust: Pour filling into crust and bake for 45-60 minutes until the filling is set and the top is firm. If the edges of the crust are browning too quickly, cover with a pie crust shield or aluminum foil.
Cool: Let the quiche cool for at least 10 minutes before cutting. Quiche can be served warm or cold. The filling will firm up more as it cools.
Notes
1) Par-bake crust: Pie weights/dried beans/sugar are usually recommended when par-baking pie crust to prevent the crust from shrinking. However, if you're feeling lazy you can just poke holes on the surface of the crust with a fork. To minimize crust shrinkage, make sure your crust is very cold immediately before baking. You may only need to bake the crust for 10-15 minutes, just until the edges are golden.