Preheat your oven to 350°F and move the rack to the upper third. At the same time, grate your cheeses and keep them ready for layering.
Warm the olive oil in your skillet and sauté the chopped onion until soft. Add garlic and ground beef, breaking it up as it cooks. Once brown, season with salt and pepper.
Stir 2 cups of marinara sauce into the cooked beef and let it warm through. You should have a thick, hearty meat sauce.
Spread the remaining 1 cup of marinara on the bottom of the baking dish. Add half the ravioli in an even layer. Spread on half the meat sauce and top with half the mozzarella and Parmesan.
Repeat the layering: ravioli, meat sauce, mozzarella, and Parmesan. Make sure the top is well-covered with cheese.
Cover with foil and bake for 30 minutes. Remove the foil and broil briefly until the top becomes golden and bubbly. Serve hot for the best cheesy texture.