Preheat oven to 350°F. Grease an 8x8 inch casserole dish with cooking spray.
Layer the fully-cooked chicken in the bottom. Dump in the marinara sauce and mix with the chicken. Next, top with both cheeses until all the chicken is covered.
In a small bowl, stir together the breadcrumbs, olive oil, fresh parsley, and a pinch of salt and pepper.
Sprinkle the seasoned breadcrumbs over the top. (Freezing instructions begin here.)
Bake for about 20-25 minutes or until golden on top and bubbling on the sides. If you want it a little more browned on top, turn on the broiler for 1-2 minutes (watch closely so it doesn't burn!). Serve "as is" or over your favorite cooked pasta or rice.
Freeze For Later: Follow Steps 1-4. Cover the casserole tightly with a few layers of plastic wrap and/or foil, squeezing out as much air as possible. Label and freeze for up to 3 months.
Prepare From Frozen: Thaw in the fridge over night and follow Step 5. Note: If it's still slightly frozen in the middle, you'll just have to cook it longer than the recipe calls for or defrost it a bit in the microwave before baking. Cover it with foil if the top gets too brown in the oven.