• Step 1: Preheat your oven to 375°F (190°C). Grease a 9×13 inch baking dish with nonstick cooking spray or olive oil.
• 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. Add the minced garlic and sauté for an additional minute, until fragrant.
• Step 3: Stir in the diced red bell pepper, black beans, pinto beans, and corn. Cook for 2-3 minutes, until the vegetables are slightly softened.
• Step 4: Add the diced tomatoes, chili powder, cumin, oregano, and salt. Stir to combine and let the mixture simmer for 5-7 minutes, allowing the flavors to meld.
• Step 5: Spread a thin layer of the vegetable mixture in the bottom of the prepared baking dish. Top with a layer of corn tortillas, overlapping them slightly. Spread half of the remaining vegetable mixture over the tortillas, then sprinkle with half of the shredded cheese.
• Step 6: Repeat the layers of tortillas, vegetable mixture, and cheese. Finish with a final layer of tortillas and cheese on top.
• Step 7: Bake the casserole for 25-30 minutes, or until the cheese is melted and bubbly. Allow it to cool for 5-10 minutes before serving.