Pour a very thin layer of marinara sauce into the prepared baking dish to cover bottom (about ½ cup).
Arrange half of the frozen ravioli in a single layer over the sauce.
Top with half of the remaining marinara sauce and half of the mozzarella cheese.
Repeat layers once, starting with ravioli. Sprinkle Parmesan cheese on top.
Cover with aluminum foil and bake for 20 minutes. Remove foil and bake, uncovered, for 20-25 more minutes or until bubbly and hot in center. Let stand 10 minutes before serving.
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Notes
Add spinach for some extra veggies in the dish.
Instead of the vegetarian ravioli casserole, appeal to meat-lovers with some browned Italian sausage or ground beef in the sauce.
If you're serving a smaller family (or cooking just for 2), cut all of the ingredients in half and prepare the casserole in an 8-inch square pan. The rest of the cooking instructions remain the same, although you may be able to shorten the total baking time by a few minutes.
Prepare two smaller casseroles in 8-inch square pans. Bake one now, and freeze the second for a later meal!
Store leftover casserole in an airtight container in the refrigerator for 3-4 days.