If using traditional lasagna noodles, cook them in well-salted water according to package directions. Drain and set aside.
In a large skillet, heat the oil over medium heat. Add the onions and cook for 5-7 minutes until softened. Stir in the green chiles.
Add the chicken and 1¼ cups of enchilada sauce. Stir to combine. Heat for 1-2 minutes.
Coat a 9x9 baking dish with cooking spray. Place 3-4 lasagna noodles to cover the bottom of the dish. Spoon half of the chicken mixture over the top. Sprinkle ⅓ of the cheese over the chicken.
Layer 3-4 more lasagna noodles on top of the cheese. Top with the remaining chicken mixture and another ⅓ of the cheese. Top with 3-4 more noodles. Cover with the remaining enchilada sauce and the rest of the cheese.
Bake for 30-40 minutes until golden and bubbly.
Notes
Make-ahead: Assemble ahead of time and bake when ready. This helps with those busy nights when you need dinner in a flash. If refrigerated, allow 10-15 minutes of extra baking time unless the casserole is brought to room temperature first. Freezing: To freeze the unbaked casserole, cover it with foil and wrap it tightly with plastic wrap. Then, thaw before baking.