1. Preheat your oven to 375°F. Grease a 9×12-inch casserole dish generously and set it aside.

2. In a large skillet, heat the olive oil over medium heat. Add the chopped onion and a pinch of salt, and cook until the onion has softened. Stir in the minced garlic, chili seasoning, and cumin, and cook for 1 more minute. Add the shredded cooked chicken and stir until well combined. Remove from heat.

3. In a large bowl, mix together the salsa verde and sour cream or Greek yogurt until smooth and creamy.

4. Spread 3/4 cup of the salsa verde mixture in the bottom of the prepared casserole dish. Lay 8 tortilla halves over the sauce. Add half of the chicken mixture, then 1 cup of the salsa verde sauce, and sprinkle 1/2 cup of the Monterey Jack cheese over the top.

5. Repeat this layering process one more time with the remaining chicken mixture, 1 cup of salsa verde sauce, and 1/2 cup of cheese. For the final layer, add the last 8 tortilla halves, the remaining salsa verde sauce, and the rest of the Monterey Jack cheese.

6. Bake for 15 to 20 minutes, or until the cheese is melted and the sauce is bubbling around the edges. Serve hot with optional toppings.