Preheat the oven to 400°F (200°C) and lightly grease a deep 9x13-inch baking dish with 1 teaspoon of the olive oil.
Heat a large skillet over medium-high heat and add 1–2 teaspoons olive oil. Add the Italian sausage (or ground beef/turkey) and Italian seasoning, breaking the meat into bite-sized pieces as it browns.
When the meat is browned, remove from heat and drain excess grease. Stir in the 28 ounces of marinara or spaghetti sauce until combined. If omitting meat, use the prepared sauce as-is.
Spread a thin layer of sauce on the bottom of the prepared baking dish. Arrange half of the ravioli in a single layer over the sauce, overlapping slightly if needed.
Spoon half of the remaining sauce over the ravioli and spread to cover. Sprinkle half of the shredded mozzarella evenly on top.
Repeat with the remaining ravioli, sauce, and mozzarella, then sprinkle the 1/4 cup Parmesan evenly over the top.
Cover the dish tightly with foil and bake for 20 minutes. Remove foil and bake an additional 10 minutes if using fresh or thawed ravioli, or 20 minutes if using frozen ravioli.
Let the casserole rest for 10–15 minutes before serving and sprinkle additional Parmesan if desired.
