Preheat the oven to 350°F. Use a fork to prick the unbaked pie crust several times (both on the bottom and the sides), then place on a baking tray and into the preheated. Bake for 10 to 12 minutes, until the crust is partially baked and just starting to brown. Remove from the oven and set aside until needed.

Meanwhile, make the tofu-cashew custard. Combine the soaked cashews, tofu, nutritional yeast, mustard, turmeric, and cornstarch in the bowl of a food processor, and process until as smooth as possible. Set aside while you prepare the sautéed vegetables.

Heat a large sauté pan over medium-high heat, then add the oil. When hot, add in the shallots and a pinch of salt, then cook until mostly translucent, about 5 minutes. Add in the sliced mushrooms and cook for an additional 5 to 10 minutes, until the mushrooms have released their liquid and are softened. Add in the garlic, then cook an additional 3 to 4 minutes, until fragrant. Stir in the spinach, crumbled sausage, thyme, salt, and pepper, and cook until heated through. Remove from the heat.

Stir the tofu-cashew custard into the vegetable-sausage mixture, mixing well to make sure everything is well-incorporated. Pour the mixture into the par-baked pie shell, smoothing the top with the back of a spoon or spatula, and use the baking tray to transfer the quiche to the oven.

Bake for 40 to 50 minutes, until the custard is set and the crust is golden brown. If the crust browns before the filling is ready, cover the edges with strips of foil to prevent burning. Remove from the oven, allow to cool for 10 minutes, then slice and serve.