Preheat your oven to 350°F (177°C).
Cook rigatoni pasta according to package directions until just al dente, then drain and set aside.
Heat olive oil in a large pan over medium-high heat. Add diced onion and cook until softened.
Add garlic to the pan and cook until fragrant.
Add ground beef to the pan, break up chunks, and cook until no pink remains.
Stir marinara sauce into the browned beef and heat through. Season with salt and pepper, then remove from heat.
Mix the beef sauce with the cooked rigatoni pasta thoroughly.
Transfer the mixture to a 13x9 inch baking dish and cover loosely with foil.
Bake covered for 20 minutes at 350°F (177°C).
Remove foil, sprinkle mozzarella cheese evenly over the top, and bake an additional 10 minutes until cheese is melted and golden.
Garnish with fresh basil before serving.