• Step 1: Preheat your oven to 375°F (190°C).
• Step 2: In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté for 5-7 minutes, until translucent.
• Step 3: Add the minced garlic and sauté for an additional minute, until fragrant.
• Step 4: Stir in the diced red bell pepper and sliced mushrooms. Cook for 5-7 minutes, until the vegetables are tender.
• Step 5: Add the drained and rinsed black beans and the diced tomatoes (with their juices) to the skillet. Stir to combine and let the mixture simmer for 5 minutes.
• Step 6: Pour the enchilada sauce over the vegetable mixture and stir to coat evenly.
• Step 7: Grease a 9×13-inch baking dish. Arrange a layer of the tortilla quarters in the bottom of the dish, then top with half of the vegetable-enchilada sauce mixture.
• Step 8: Sprinkle half of the shredded cheese over the top, then repeat the layers of tortillas, vegetable-enchilada sauce, and cheese.
• Step 9: Bake the casserole in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
• Step 10: Remove the casserole from the oven and let it cool for 5-10 minutes before serving.