Sauté the aromatics: Heat olive oil and butter in a large deep skillet over medium heat. Add onion and a pinch of salt.
Cook 3–4 minutes until softened. Stir in garlic, Italian seasoning, and red pepper flakes. Cook 30 seconds until fragrant.
Build the sauce base: Pour in crushed tomatoes, broth, and tomato sauce.
Stir well and bring to a gentle simmer. Taste and add the sugar if the sauce is sharp. Season lightly with salt and pepper.
Add the tortellini: Stir in the tortellini so it’s mostly submerged.
Reduce to medium-low and simmer uncovered for 5–7 minutes, stirring occasionally so nothing sticks. The sauce will reduce slightly as the pasta cooks.
Make it creamy: When the tortellini is tender, lower the heat and stir in the heavy cream. Simmer 1–2 minutes to thicken slightly.
Finish with cheese and basil: Stir in Parmesan until melted and smooth.
Fold in fresh basil. If using spinach or pre-cooked add-ins, stir them in now and heat through.
Adjust and serve: Taste and season with more salt and pepper as needed. If the sauce is too thick, loosen with a splash of broth or cream.
Serve hot with extra Parmesan and basil on top.