Preheat the oven to 425°F (220°C). Spray a 9x13-inch baking dish with nonstick spray.
In a medium bowl, toss the shredded rotisserie chicken with the taco seasoning until evenly coated; set aside.
In a food processor or high-powered blender, combine the baby spinach, fresh cilantro, and salsa and pulse until finely chopped and well combined to form a green salsa mixture.
Layer six of the 6-inch tortillas in the bottom of the prepared baking dish, overlapping slightly to cover the surface.
Top the tortilla layer with 1 cup of the shredded Mexican blend cheese, half of the drained black beans, half of the drained corn, and half of the seasoned chicken. Spoon about 1 cup of the spinach-cilantro salsa mixture over the chicken.
For the second layer, repeat: a layer of tortillas, the remaining black beans, remaining corn, remaining seasoned chicken, and the rest of the spinach-cilantro salsa. Finish by sprinkling the remaining 1 cup of shredded cheese over the top.
Cover the dish tightly with foil and bake for 30 minutes, until heated through and the cheese is bubbly.
Remove the foil and bake an additional 10–15 minutes until the top is browned to your liking. Let rest a few minutes before slicing and serving.
Serve warm and top as desired with jalapeños, plain Greek yogurt or low-fat sour cream, and extra cilantro.