Begin by preheating your oven to 375°F and misting your baking dish with cooking spray. Heat olive oil in a large sauté pan over medium heat. Add the diced onion and poblano pepper, cooking until softened, about 6-7 minutes. Stir in the beans, corn, spinach, chicken, and 2 cups of enchilada sauce. Cook for another 2-3 minutes to combine the flavors, then remove from heat.
Spread ½ cup of the remaining enchilada sauce evenly on the bottom of the baking dish. Add a layer of tortilla halves to cover the surface. Spoon one-third of the filling mixture over the tortillas, followed by one-third of the cheese. Repeat the layers until all the ingredients are used, finishing with a layer of cheese and sauce on top.
Cover the dish with aluminum foil and bake for 20 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden. The aroma filling your kitchen at this point is incredible!
Once out of the oven, allow the casserole to rest for 5 minutes. This helps it set up so it’s easier to serve. Add your favorite toppings right before serving for added freshness and flair.