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 the minced garlic and cook for 1 minute until fragrant.
Stir in the shredded chicken, chopped spinach, ground cumin, chili powder, salt, and black pepper. Cook for 4-5 minutes until the spinach is wilted and the mixture is heated through. Remove from heat.
Pour 1 cup of the enchilada sauce into the bottom of a 9×13-inch baking dish and spread evenly.
Warm the corn tortillas in a microwave or on a skillet until pliable.
Spoon about 1/4 cup of the chicken and spinach mixture onto each tortilla, roll it up tightly, and place seam-side down in the baking dish.
Pour the remaining 1 cup of enchilada sauce evenly over the rolled tortillas.
Sprinkle the shredded Monterey Jack cheese evenly over the top.
Cover the baking dish with aluminum foil and bake for 20 minutes.
Remove the foil and bake for an additional 10 minutes until the cheese is melted and bubbly.
Let the enchiladas cool for 5 minutes before serving.