Cook the tortellini in a pot of boiling salted water according to package instructions until al dente. Drain and set aside.

Preheat the oven to 180°C (350°F).

In a skillet, heat olive oil over medium heat. Add chopped onion and sauté until translucent.

Add minced garlic, dried oregano, salt, and black pepper. Cook for another 2 minutes.

Stir in the tomato sauce and bring to a simmer. Cook for 5 minutes.

In a large baking dish, combine the cooked tortellini and tomato sauce. Mix well.

Top with cubed mozzarella and grated Parmesan cheese.

Bake in the preheated oven for 20 minutes, or until the cheese is bubbly and golden.

Remove from the oven and let it cool for a few minutes before serving.

Garnish with fresh basil before serving.