Preheat your oven to 425°F and spray an 8-inch square baking dish with cooking spray.
In a large bowl, whisk together the condensed soup, milk, chicken broth, salt, pepper, and garlic powder until well combined.
Add the finely diced onion, sliced mushrooms, red bell pepper, celery, raw chicken pieces, uncooked penne pasta, and half of the grated cheddar cheese. Stir gently to coat all ingredients.
Pour the mixture into the prepared baking dish and spread it out evenly. Cover tightly with foil and bake for 45 minutes.
After 45 minutes, remove the foil and stir the casserole. Check the pasta's texture; it should be tender but still have a slight bite. If needed, re-cover and bake for a few more minutes.
Once the pasta is cooked, sprinkle the remaining cheddar cheese on top. Bake uncovered for an additional 5 to 10 minutes until the cheese is melted and bubbly.
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Notes
Store leftovers in an airtight container in the fridge for 3 to 4 days. Reheat in the microwave or oven.