Preheat oven to 400 degrees F.
Bake pie shell for 10 minute or until crust is golden brown.
Steam baby spinach (fresh or frozen) for about 5 minutes. Let cool and squeeze out as much liquid as possible.
Saute onions for 2 minutes until translucent and add sliced mushrooms. Continue to saute for another 2 - 3 minutes until mushrooms are cooked.
In a large bowl, whisk together mayonnaise and almond milk until smooth.
Beat eggs, then whisk them into the mayonnaise/almond milk mixture. Set aside.
Layer the cheese substitute into the pie shell first.
Layer spinach, onion and mushroom on top of the cheese substitute.
Pour egg mixture over the top.
Place quiche on a cookie sheet and cover quiche with foil.
Bake in preheated 400 degree oven for around 45 minutes. Remove the foil, and bake for an additional 15 minutes. The top should be golden brown and filling set.
Let cool for about 10 minutes and serve.