1 hr 15 min
In a very large, nonstick skillet, heat oil over medium-high.
Add onion, garlic, and peppers and stir. Add mushrooms and cook until the liquid evaporates (15-20 minutes). If the pan is not large enough, cook mushrooms in batches.
Pour half of the enchilada sauce into a 13x9-inch baking dish.
Arrange 8 tortilla halves over the sauce in the baking dish. You'll use the other 8 tortilla halves later.
Spoon half the mushroom mixture into the baking dish, carefully covering tortillas.
Set aside 1/4 cup (about 1 ounce) of grated cheese. Sprinkle the rest of the cheese on top of the mushroom mixture.
Layer with the remaining 8 tortilla halves, then the remaining mushroom mixture, and enchilada sauce. Sprinkle top of casserole with the remaining 1/4 cup cheese.
Cover baking dish with foil and bake until casserole is steaming hot in the center (about 30 minutes).
Allow casserole to cool for 5 minutes before serving.
Serve with pinto or black beans and salad.