Preheat the oven to 425 degrees F.
In a food processor, finely chop the cashews.
Add the almond milk, tahini, nutritional yeast, lemon juice, ½ teaspoon of the salt, and garlic powder.
Blend until the mixture is the consistency of creamy peanut butter, adding water as needed.
Set aside.
Slice the beets into ¼-inch-thick slices (a mandolin works well for this).
Cut the portobellos into large ¼-inch-thick slices.
Drizzle 1 tablespoon of the oil in a 9-by-12-inch baking dish.
Arrange one layer of beets along the bottom of the dish.
Arrange a layer of mushrooms over the beets.
Spread half of the cashew mixture over the mushrooms.
Arrange another layer of beets on top of the nut spread and repeat the layering ingredients in this order (it’s best to end with a layer of mushrooms).
Drizzle the remaining 1 tablespoon oil over the top layer and sprinkle with the remaining 1 teaspoon salt.
Cover and bake for 55 minutes.
Serve warm.