Preheat oven to 350 degrees and spray broiler pan or baking sheet with cooking spray. Place ground chicken, breadcrumbs and Parmesan in a large mixing bowl and set aside.
Mince onion, bell pepper, mushrooms and garlic very finely or pulse in food processor until small but not pureed.
Add minced vegetables, beaten egg, salt, pepper, oregano, basil and red pepper flakes to the bowl containing ground chicken. Mix by hand until completely combined.
Transfer to the prepared pan and shape into a loaf about 5" wide and 2" high. Bake 30 minutes then remove from oven. Spoon marinara sauce over meatloaf and spread to edges.
Return to oven for additional 25–30 minutes until cooked through or internal temperature registers 160 degrees on meat thermometer. Remove from oven and sprinkle with Italian blend cheese. Allow meatloaf to rest 10 minutes before slicing and serving.
If you want to make it spicy, use jalapeños instead of green bell peppers!