Start by making the sauce. In a medium pot, bring 2 cups of water to a boil. Place the raw cashews into a small bowl and pour the boiling water over them, so the cashews are fully covered. Allow the cashews to soak for at least 30 minutes.
While the cashews soak, begin working on the casserole. Place the vegetable broth into the same pot that you used to boil the water for the cashews, and bring it to a boil. Add the wild rice, then reduce to a simmer and cook for 30 minutes. Remove from the heat, cover with a lid and allow to sit for 15 minutes. While you’re waiting for the cashews to soak and the rice to cook, you can start prepping your vegetables.
Preheat the oven to 350°F then lightly grease a large (about 4-QT) casserole dish. In a large pot, warm 4 tablespoons vegan butter over medium heat. Add the onion, salt and pepper, then cook for 3 minutes, until the onions are translucent. Next, add the mushrooms, garlic, carrot, and celery, then stir to combine. Cook for a few minutes, then add the tamari, thyme, and parsley. Stir again while continuing to cook for about 5 minutes.
Once the cashews are done soaking, finish the sauce by first straining off the excess water and placing the cashews into a high speed blender. Add the almond milk, nutritional yeast, and arrowroot powder, then blend on high for 30 seconds, until smooth. Season with salt to taste. Pour the mixture into the pot with the vegetables and stir. Bring the cashew mixture to a boil, then reduce to a simmer and cook for 10 minutes. Turn the heat off, then add the cooked rice to the pot. Stir together until combined. Season with salt to taste.
Transfer the mixture to the casserole dish then smooth down with the back of a spoon so that the surface is even. Top with breadcrumbs, cover with foil and bake in the oven for 25 minutes. Remove casserole from the oven, uncover and allow to cool for at least 15 minutes before serving. Top with fresh parsley, serve and enjoy!