Preheat oven to 400 degrees and grease a 9×13 baking dish with oil.
Heat a large skillet over medium high heat. Once hot, add the avocado oil and mushrooms. Sauté for 4-6 minutes until the mushrooms are golden brown and crispy.
Next, add the diced onion, bell peppers, minced garlic, and zucchini. Sauté again for another 4-6minutes until tender and then remove from the heat. Stir in the baby spinach and set aside.
Make the Filling: Add the sautéed veggies, corn, black beans, lime juice, hatch chilies, Greek yogurt, 1 cup of enchilada sauce, cumin, and red pepper flakes to a mixing bowl. Stir until everything is well combined.
Assemble the Casserole: Coat the bottom of the 9×13 baking dish with 1/3 cup of enchilada sauce. Then place a layer of 4 tortillas in the dish (they will overlap some). Next, add approximately a heaping cup of the veggie filling and spread out with a spatula or the back of spoon. Next, add a layer of pepper jack cheese. Then pour on 1/3 cup of enchilada sauce. Repeat the process until all of the tortillas and pepper jack are used, you will have 3 layers ending with the veggie filling on top.
Pour the remaining enchilada sauce over the casserole and then sprinkle with an even layer of Monterrey cheese over the casserole.
Bake at 400 degrees for 20-25 minutes until cheese is melty, bubbly, and golden.
Garnish the enchilada casserole with chopped cilantro, diced red onion, sliced avocado, sliced jalapeño, sour cream or Greek yogurt, and fresh limes.