In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until softened.
Stir in the minced garlic, diced carrots, and diced celery. Cook for an additional 5 minutes until the vegetables are tender.
Add the dried oregano, dried basil, and red pepper flakes (if using) to the pot. Stir well to combine.
Pour in the vegetable broth and diced tomatoes, bringing the mixture to a boil.
Once boiling, reduce the heat to a simmer and add the cheese tortellini. Cook according to package instructions, usually about 5-7 minutes.
Stir in the fresh spinach and cook for another 2-3 minutes until wilted. Season with salt and pepper to taste.
Serve hot, garnished with grated Parmesan cheese.