Preheat the oven to 400°F. Move a rack to the lowest position in the oven.

Prick the bottom of the frozen pie shell several times with a fork, and transfer to a baking sheet. If using a refrigerated crust, arrange it in a 9-inch pie pan, then prick the bottom with a fork.

Bake the pie shell on bottom rack of oven until the bottom is slightly puffed, 12-14 minutes, or according to package directions. When done, remove from oven and set aside.

Reduce oven temperature to 350°F.

Slice the mushrooms, finely dice the red pepper, and roughly chop the spinach.

Set a large skillet over medium-high heat and add the olive oil. Once the oil is shimmering, add the red pepper and mushrooms. Cook, stirring frequently, for 1-2 minutes, then add the spinach. Season lightly with salt and pepper and continue to cook until the spinach is fully wilted and the mushrooms are browned, 1-2 minutes. Remove the skillet from the heat and set aside.

Place the tofu, cashew milk, nutritional yeast, turmeric, garlic powder, salt, and black pepper in a blender or food processor. Blend on high until completely smooth, about 1 minute. Transfer the mixture to a large mixing bowl.

Add the spinach, mushrooms, and peppers to the tofu mixture. Fold with a rubber spatula until the vegetables are evenly distributed.

Pour the tofu mixture into the pie shell. Cover the edges of the pie shell with pie guards or strips of aluminum foil to avoid burning or over-browning the crust.

Bake the quiche on bottom rack of oven until the top is firm and golden brown, 55-65 minutes.

Check to see that quiche is done. Remove from oven or add time as needed.

Allow to cool for at least 30 minutes at room temperature before slicing and serving. Leftover quiche may be stored covered in the refrigerator for up to 3 days.