Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: In a large skillet or saucepan, heat the olive oil over medium heat. Add the diced onion and cook until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant. Stir in the drained diced green chilies.
Step 3: Add the shredded rotisserie chicken to the skillet with the onion mixture. Pour in approximately 1.5 cups (half of the can) of the red enchilada sauce. Stir everything together until the chicken is well coated and heated through.
Step 4: Spread a thin layer of enchilada sauce (about 1/4 cup) on the bottom of your prepared baking dish.
Step 5: Tear 6 of the corn tortillas into bite-sized pieces and scatter about half of them over the sauce in the baking dish.
Step 6: Spread half of the chicken mixture evenly over the tortilla pieces.
Step 7: Sprinkle about 1 cup of shredded Mexican blend cheese over the chicken layer.
Step 8: Repeat the layers: add the remaining torn tortilla pieces, then the rest of the chicken mixture, followed by another cup of shredded cheese.
Step 9: Pour the remaining enchilada sauce (about 1.5 cups) evenly over the top of the casserole, ensuring good coverage.
Step 10: Finish with the last cup of shredded cheese, scattering it evenly over the top.
Step 11: Bake uncovered for 20-25 minutes, or until the cheese is bubbly and golden brown, and the sauce is simmering. If the cheese starts to brown too quickly, you can lightly tent the dish with foil.
Step 12: Remove from the oven and let it rest for 5-10 minutes before serving. Garnish with fresh cilantro and a dollop of sour cream or Greek yogurt, if desired. Serve warm and enjoy!