Preheat the oven to 375°F (190°C) and grease a 9x12-inch casserole dish.
Heat olive oil in a large skillet over medium heat, then sauté chopped onion with a pinch of salt until softened.
Add minced garlic, chili seasoning, and ground cumin to the skillet; cook for one minute until fragrant.
Stir in shredded cooked chicken until well combined, then remove from heat.
In a large bowl, mix together salsa verde and sour cream until smooth and creamy.
Spread ¾ cup of the salsa verde mixture evenly in the bottom of the casserole dish.
Layer 8 tortilla halves over the salsa, then add half the chicken mixture, 1 cup of salsa verde sauce, and ½ cup of grated cheese.
Repeat the layering: 8 tortilla halves, remaining chicken mixture, 1 cup salsa verde sauce, and ½ cup cheese.
Top with a final layer of 8 tortilla halves, the remaining salsa verde sauce, and the last 1 cup of cheese.
Bake the casserole in the preheated oven for 15-20 minutes until cheese is melted and sauce bubbles.
Remove from oven, let cool slightly, then garnish with sliced jalapeños, crumbled cotija, diced red onion, and fresh cilantro before serving.