Preheat the oven to 375°F (190°C).
Cook the cheese tortellini according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add diced onion and minced garlic; sauté until softened and fragrant, about 3-4 minutes.
Add ground beef to the skillet, season with salt, black pepper, and Italian seasoning. Cook until browned and cooked through, breaking up the meat with a spoon.
Stir in marinara sauce and simmer for 5 minutes to combine flavors.
In a large mixing bowl, combine cooked tortellini and the beef marinara sauce mixture.
Transfer the mixture into a shallow white ceramic baking dish.
Sprinkle mozzarella and parmesan cheese evenly over the top.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted, bubbly, and golden brown on top.
Remove from oven and let rest for 5 minutes before scooping onto a round white plate.
Garnish with fresh basil leaves and serve warm.