Preheat oven to 350 degrees F. Lightly grease a 9x13 baking dish with cooking spray. Set aside.

In a large pan, heat olive oil over medium-high heat. Add the onion and cook until tender, about 3-4 minutes. Add the garlic and cook and additional two minutes. Stir in the spinach and cook until spinach wilts and shrinks down. Stir in the pasta sauce and diced tomatoes. Add the oregano, dried basil, red pepper flakes, and fresh basil. Stir and season with salt and black pepper, to taste.

Pour 1/2 cup of the sauce on the bottom of the baking dish. Add half of the tortellini to the dish and spread out evenly. Top with more sauce, just to cover the tortellini. Sprinkle 1 cup of mozzarella cheese over tortellini and sauce. Next, sprinkle 1/4 cup of Parmesan cheese over the mozzarella cheese. Add the rest of the tortellini to the dish in an even layer. Top with remaining sauce. Sprinkle the rest of the mozzarella and Parmesan cheese over the top. Garnish with fresh basil, if desired.

Spray a sheet of aluminum foil with cooking spray and cover baking dish. Bake for 30 minutes. Remove foil from pan and bake for an additional 10 minutes, or until cheese is completely melted and bubbly. Remove from oven and let sit for 5 minutes before serving.