Preheat your oven to 375°F (190°C) and grease a 9×12-inch casserole dish generously with cooking spray or butter. Set aside.

Heat olive oil in a large skillet over medium heat. Add the chopped onion and a pinch of salt, cooking for 2-3 minutes until softened and translucent.

Stir in minced garlic, chili powder, and cumin. Cook for about 1 minute, stirring constantly, until fragrant. Be careful not to burn the garlic.

Add the shredded cooked chicken to the skillet and stir until everything is well incorporated and the chicken is coated with the spice mixture. Remove from heat and set aside.

In a large bowl, mix together the salsa verde and sour cream, whisking until fully combined to create a smooth and creamy sauce.

Spread 3/4 cup of the salsa verde mixture evenly across the bottom of the greased casserole dish.

Cut tortillas in half and lay 8 tortilla halves over the salsa verde layer, overlapping slightly to cover the bottom completely.

Add half of the chicken mixture on top of the tortillas, followed by 1 cup of the salsa verde sauce, and sprinkle 1/2 cup of cheese over the top.

Repeat the layering by adding another 8 tortilla halves, the remaining chicken mixture, another 1 cup of salsa verde sauce, and 1/2 cup of cheese.

Top with a final layer of 8 tortilla halves, followed by the remaining salsa verde sauce and the rest of the cheese, ensuring the cheese covers the entire surface.

Place the casserole dish in the preheated oven and bake for 15-20 minutes until the cheese melts beautifully and turns golden in spots, and the sauce bubbles around the edges.

Remove from the oven and let the casserole rest for 10-15 minutes before serving. This allows the layers to set up for cleaner portions. Serve with optional toppings such as chopped cilantro, diced avocado, cotija cheese, red onion, or sliced jalapeños.