Preheat the oven to 375°F (190°C).
Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook for 3-4 minutes until softened.
Add minced garlic, ground cumin, chili powder, salt, and black pepper to the skillet. Cook for 1 minute until fragrant.
Add the chopped spinach to the skillet and cook until wilted, about 2-3 minutes. Remove from heat.
In a large bowl, combine the shredded chicken, cooked spinach mixture, and 1/2 cup of shredded cheddar cheese. Mix well.
Warm the corn tortillas in the microwave for 20-30 seconds to make them pliable.
Spread 1/2 cup of enchilada sauce evenly on the bottom of a 9×13 inch baking dish.
Place about 1/3 cup of the chicken and spinach filling onto each tortilla, roll it up tightly, and place seam side down in the baking dish.
Pour the remaining enchilada sauce evenly over the rolled tortillas.
Sprinkle the remaining cheddar cheese and all of the Monterey Jack cheese evenly on top.
Cover the baking dish with aluminum foil and bake for 20 minutes.
Remove the foil and bake for an additional 10 minutes or until the cheese is melted and bubbly.
Let the enchiladas cool for 5 minutes before serving.