Preheat oven to 375 degrees and coat a shallow 2-qt baking dish with cooking spray. Arrange cooked chicken into an even layer in the bottom of the dish then sprinkle with dried basil and oregano.
Pour marinara sauce over chicken and spread evenly. Sprinkle Parmesan cheese and mozzarella over marinara sauce and set aside.
In a small bowl, add breadcrumbs, olive oil or melted butter and salt. Stir until breadcrumbs are evenly moistened then scatter the mixture over top of casserole. Bake 25–30 minutes until top is nicely browned, cheese is melted and sauce is bubbly.
Remove from oven and allow it to rest 5 minutes before serving.
Don’t waste any sauce that may have accumulated in the bottom of the casserole! Spoon over pasta to serve as a side dish.