First, preheat your oven to 350°F (175°C). While it's heating up, lightly grease your 8x8 glass baking dish to prevent the ravioli from sticking. This simple step makes serving and cleanup much easier later on.
Arrange half of the frozen cheese ravioli (about 16 pieces) in a single layer at the bottom of your prepared dish. Pour half of the spaghetti sauce over them and give it a gentle stir. Make sure all the ravioli are evenly coated in the sauce.
Place the pepperoni slices in a single layer over the sauce-coated ravioli. Then, sprinkle the shredded Parmesan cheese evenly over the entire dish. This creates a savory, cheesy middle layer that adds a ton of flavor.
Place the remaining ravioli on top of the Parmesan cheese, creating another single layer. Spoon the rest of the spaghetti sauce over this top layer. Take a moment to ensure all the ravioli are well-covered with sauce to help them cook through.
Arrange the last of your pepperoni slices over the top layer of sauce. Finally, sprinkle the shredded mozzarella cheese evenly over the entire dish. This top layer of mozzarella will melt into a golden, bubbly crust.
Cover the dish with aluminum foil and bake for 30 minutes. The foil traps steam, which cooks the frozen ravioli perfectly. During the last 5 minutes of baking, you can remove the foil if you prefer a more golden-brown cheese topping.