Heat olive oil and butter in a large Dutch oven over medium-high heat. Add onion and cook 4–5 minutes until softened. Add garlic and tomato paste; cook, stirring, 1–2 minutes until the paste darkens to a rust color.
Add Italian seasoning and red pepper flakes; stir 30 seconds. Pour in crushed tomatoes, broth, and balsamic vinegar. Season with salt and pepper. Bring to a boil, then reduce to a simmer. Cook uncovered 5 minutes.
Add tortellini. Simmer 3–5 minutes (per package directions) until tender.
Reduce heat to low. Stir in heavy cream. Add spinach and stir 1–2 minutes until wilted. Taste and adjust seasoning.
Ladle into bowls. Top with Parmesan and fresh basil. Serve with crusty bread.