Preheat the oven to 375 degrees Fahrenheit (190 degrees Celsius). Combine bulgur with 2 cups of cold water in a large saucepan over medium heat. Bring to a boil, then cover and turn the heat to low. Simmer for 15 minutes until cooked.
Meanwhile, heat 1 tablespoon of olive oil in a skillet over medium-high heat. Add mushrooms and brown on one side for 60 seconds. Stir and continue to cook until soft, about 3 to 4 minutes. Remove from heat and place in a large bowl.
Once the bulgur has cooked, fluff it with a fork and transfer it to the large bowl. Add the black beans, ¼ of the enchilada sauce, and ½ of the shredded cheese. Fold them together with a spatula.
Pour another ¼ of the enchilada sauce into the bottom of a 13" x 9" baking dish. Arrange five tortillas to cover the bottom (you may need to cut into quarters). Spoon half of the filling over the top and press down with a spatula. Pour another ¼ of the enchilada sauce over the top.
Arrange another five tortillas in the dish to create a second layer, followed by the second half of the filling and another ¼ of the enchilada sauce.
Arrange the final five tortillas over the top of the dish, then pour the remaining enchilada sauce over the top. Spread the remaining shredded cheese over the top of the dish. Cover with well-greased foil and bake for 25 minutes. Remove the foil and bake for another 5 minutes or until the cheese is bubbly. Let rest for 15 minutes, top with desired toppings, and serve.
Notes
Store the Black Bean Enchilada Casserole in an airtight container in the refrigerator for up to 4 days. To freeze, wrap portions tightly, and it will stay good for up to 3 months.To reheat, warm in the oven at 350 degrees Fahrenheit (175 degrees Celsius) for 15-20 minutes until thoroughly heated, or microwave individual portions in 30-second intervals, stirring between intervals until hot.