1. Preheat the oven to 350-375°F to prepare for baking.
2. In a skillet, brown the Italian sausage or ground beef with chopped onions until cooked through.
3. Prepare a 9×13-inch baking dish for assembling the casserole.
4. Begin layering by adding a layer of cheese ravioli at the base of the dish.
5. Spread a layer of the browned meat and onion mixture over the ravioli.
6. Pour a generous ladle of marinara sauce evenly over the meat layer.
7. Repeat the layering process with ravioli, meat mixture, and marinara sauce until all ingredients are used.
8. Top the final layer with mozzarella cheese for a gooey, cheesy crust.
9. Optionally, add other cheeses like provolone or cheddar for added flavor.
10. Place the assembled casserole in the oven and bake for 20-35 minutes or until bubbly and golden on top.
11. For a crispy cheese topping, broil the casserole for 1-2 minutes, ensuring it doesn’t burn.
12. Remove the casserole from the oven and allow it to cool for a few minutes before serving.
13. Serve the ravioli casserole warm and enjoy its cheesy, savory flavors.
14. If desired, store leftovers in an airtight container in the refrigerator or freeze for later use.