Cook the tortellini according to package directions. Drain and set aside.
While the tortellini is cooking, heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook until softened, about 5 minutes.
Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.
Pour in the crushed tomatoes. Stir in the heavy cream, Parmesan cheese, dried basil, dried oregano, and red pepper flakes (if using).
Bring the sauce to a simmer and cook for 5-7 minutes, stirring occasionally, until slightly thickened.
Season the sauce with salt and black pepper to taste.
Add the cooked tortellini to the sauce and toss to coat. Cook for another 2-3 minutes until heated through.
Serve immediately, garnished with fresh basil leaves.
Notes
For a lighter sauce, you can substitute half-and-half for heavy cream.