In a food processor, add the walnuts and mushrooms. Pulse until chopped well, about the consistency of cooked ground meat. Set aside.
In a large skillet, heat the olive oil. Add the onion and cook until translucent and lightly browned, about 2-3 minutes. Add the garlic and cook another minute.
Add the mushrooms, walnuts, and zucchini. Cook for 5-7 minutes or until liquid from mushrooms is cooked off and zucchini begins to soften.
Add the green chiles, cumin, salt, pepper, and black beans. Stir well to combine and cook another minute then turn off the heat. Combine cheeses in medium bowl; mix well.
In the bottom of an 8″x8″ pan, spread about ¼ cup worth of enchilada sauce. Top with a layer of tortilla halves. Top with ½ of the mushroom/zucchini mixture. Add ⅓ of the cheese. Top with ½ of the remaining enchilada sauce.
Repeat by layering tortillas, mushroom/zucchini mix, and cheese. Top with another layer of tortillas and pour the rest of the enchilada sauce over. (May need to tap casserole on counter gently to allow sauce to flow down into casserole.)
Top with remaining ⅓ of the cheese. Place a sheet of aluminum foil that has been sprayed with cooking spray over top of the casserole and bake for 20 minutes.
Remove the foil and bake another 10 minutes. Allow to cool 10 minutes before cutting and serving.
