Set your oven to heat at 375°F (190°C) and grease a 9×12-inch baking dish well.
Warm olive oil in a skillet over medium heat, then add the chopped onion and a pinch of salt, cooking until softened.
Add the minced garlic, chili seasoning, and cumin to the skillet and let the spices toast for about a minute.
Stir in the shredded chicken until everything is heated and combined, then remove from the heat.
In a separate bowl, whisk together the salsa verde and sour cream until smooth and creamy.
Spread ¾ cup of the salsa verde sauce evenly on the bottom of the prepared baking dish.
Arrange 8 halves of corn tortillas over the sauce layer.
Top the tortillas with half of the chicken mixture, then add 1 cup of the salsa verde sauce and sprinkle with ½ cup of Monterey Jack cheese.
Repeat the layers by adding the next 8 tortilla halves, followed by the rest of the chicken, 1 cup more of salsa verde sauce, and another ½ cup of cheese.
Finish with the last 8 tortilla halves, pour the remaining salsa verde sauce on top, and cover with the last 1 cup of cheese.
Bake the casserole for 15-20 minutes until the cheese melts nicely and the sauce bubbles around the edges.
Let the dish cool for a few minutes after baking, then garnish with sliced jalapeños, crumbled cotija cheese, diced red onion, and chopped fresh cilantro before serving.