Preheat the oven to 425°F (220°C) and spray a 9x13-inch baking pan with nonstick spray.
In a medium bowl, combine the shredded rotisserie chicken and the taco seasoning until evenly coated; set aside.
In a food processor or high-powered blender, pulse the baby spinach, fresh cilantro, and salsa until finely chopped and combined into a green mixture.
Arrange 6 of the 6-inch tortillas in a single layer in the bottom of the prepared pan, overlapping slightly.
Sprinkle 1 cup of the shredded cheese over the tortillas, then evenly add half of the drained black beans, half of the drained corn, and half of the seasoned chicken. Spread half of the spinach‑cilantro salsa mixture over the chicken.
Add a second layer of tortillas, then top with the remaining black beans, remaining corn, remaining seasoned chicken, and the remaining spinach‑cilantro salsa mixture. Finish with the remaining 1 cup of shredded cheese.
Cover the pan with foil and bake for 30 minutes, until the cheese is bubbly.
Remove the foil and bake an additional 10–15 minutes until the top is browned to your liking.
Let the casserole rest a few minutes, then slice and serve warm with optional jalapeños, plain Greek yogurt or low-fat sour cream, and extra cilantro.