Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and minced garlic, and sauté until softened and fragrant, about 5 minutes.
Add the chopped fresh spinach to the skillet and cook until wilted, about 2-3 minutes.
In a large mixing bowl, combine the shredded chicken, sautéed onion and spinach, condensed cream of chicken soup, sour cream, salsa, ground cumin, chili powder, salt, and pepper. Mix until well combined.
Spread a layer of the chicken mixture on the bottom of the greased baking dish.
Top with a layer of tortilla quarters, followed by a layer of shredded cheddar cheese and Monterey Jack cheese.
Repeat the layers until all the ingredients are used, ending with a layer of cheese on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool slightly before serving.
Garnish with fresh cilantro and enjoy your Chicken Tortilla Spinach Casserole!