01 - Heat olive oil in a large nonstick skillet over medium heat. Add diced onion and sauté until soft, about 3 minutes. Stir in garlic and cook until fragrant, 1 minute.
02 - Fold in shredded chicken and chopped spinach. Cook, stirring occasionally, until spinach wilts, approximately 2 minutes. Remove mixture from skillet and set aside.
03 - In a bowl, combine enchilada sauce with sour cream, cumin, chili powder, kosher salt, and black pepper. Mix until smooth.
04 - Coat the skillet bottom with a small amount of sauce. Layer 4 flour tortillas over sauce, overlapping as needed.
05 - Spread half the chicken-spinach mixture over tortillas. Top with a third of the sauce and a third of the shredded Monterey Jack cheese.
06 - Layer the remaining tortillas. Add the rest of the chicken mixture, another third of the sauce, and another third of cheese.
07 - Spoon remaining sauce over the top and sprinkle with the last of the cheese. Cover and let simmer on medium-low heat until cheese is melted and mixture is heated through, about 8–10 minutes.
08 - Remove skillet from heat. Sprinkle chopped cilantro over prepared enchiladas. Slice and serve warm.