Heat oil in a large skillet over medium-high heat. Add onion, poblano pepper, chicken, chili powder, cumin, garlic powder, and salt. Stir and cook for approximately 8 minutes or until chicken is cooked through.
While chicken cooks, preheat oven to 375 degrees F. Spray an 8 x 11″ baking dish with cooking spray. In a medium bowl, whisk together sour cream and salsa.
Now you’re ready to assemble! Place a single layer of tortillas in the bottom of the prepared baking dish. Spread half of the chicken and peppers over the tortillas. Drizzle about a third of the sour cream mixture over the chicken, and then spread a handful of cheese over the sour cream.
Repeat the layers in step three once more. Finally, arrange one more layer of tortillas on top. Spread the rest of the sour cream over the tortillas, and spread another handful of cheese over that.
Cover casserole with foil, and bake in preheated oven for 30 minutes. Remove foil, increase heat to 400 degrees F, and bake an additional 5-10 minutes until top is brown and bubbly. (You can optionally spread a little more cheese over the top for this final baking step.)
Allow to set for a few minutes before cutting and serving. Garnish with sliced green onions if desired.
If you don’t have an 8 x 11″ pan, you can use a 9 x 13″ pan. The casserole will just be a bit flatter.