Preheat your oven to 350°F (175°C) and lightly spray a baking dish with cooking spray.
Heat a large skillet to medium heat and lightly spray with olive oil.
Add the poblano peppers, onion, garlic, and saute until peppers begin to soften, about 10 minutes.
Once the peppers are softened, spoon flour over the peppers to coat and cook for another 2 minutes.
To the skillet add the milk and chicken stock and gently bring to a bubble and add a handful of cilantro.
Using an immersion blender, blend the fresh peppers until the sauce is creamy and smooth.
Once the mixture is smooth, add one wedge of pepper jack cheese and stir until the cheese melts. Season the sauce with salt and black pepper to taste.
In the baking dish, coat bottom of the dish with about a 1/4 cup of the creamy sauce and layer with 6 corn tortillas.
Spread half of the shredded chicken mixture evenly over the tortillas, followed by a sprinkle of shredded Monterey Jack cheese and another 1/4 of the poblano cream sauce.
Top with another layer of 6 tortillas, pour the remaining creamy poblano sauce over the top layer of tortillas and sprinkle with additional cheese.
Bake for 15-20 minutes or until the top layer of cheese is melted, bubbling and golden brown.
Once baked, remove the casserole from the oven and let it cool slightly.