Preheat oven to 350˚F. Add 1 tablespoon of oil to a 10” quiche or pie dish and generously coat the bottom and sides. Set aside.
Add the remaining 1 tablespoon of oil to a large skillet and heat over medium heat. Add the diced onion and ½ teaspoon of salt. Sauté until translucent, 3-5 minutes.
Add the mushrooms and sausage. Sauté for another 4-5 minutes.
Lastly, add in the chopped spinach and sauté until wilted. Remove from heat. Add salt and black pepper to taste and set aside.
In a high-speed blender, add the tofu, vegan mozzarella shreds, non-dairy milk, chickpea flour, nutritional yeast, remaining ½ teaspoon of salt, and turmeric. Blend until completely smooth. There should be no chunks at all, and it should resemble a custard. Set aside.
Add the vegetable and sausage mixture to the prepared quiche dish and spread it out evenly. Next, pour the custard mixture and carefully mix it in the dish until everything is evenly distributed. Smooth the top with the back of a spatula.
Bake for 45 minutes to 1 hour, depending on how custard-like you want it.
After baking, remove from oven and allow to cool for at least 15 minutes before slicing. The longer it cools, the easier it will be to slice.
I haven't personally baked this vegan quiche filling in a pie crust, but it would likely work well. If you would like to experiment with baking in a buttery crust, I would recommend blind baking the crust at 375F for about 22-25 minutes, removing from the oven halfway in between to prick holes in the bottom.