In a large pot, heat olive oil over medium heat. Add diced onion and cook for 3-4 minutes until softened. Add minced garlic and cook for another minute.
Pour in the leftover spaghetti sauce and chicken broth, stirring well to combine. Bring to a gentle simmer.
Add the tortellini and cook according to the package instructions (usually 5-7 minutes for fresh, 8-10 for frozen).
Stir in baby spinach, if using, and let it wilt for 1-2 minutes.
Season with Italian seasoning, salt, and pepper to taste.
Serve hot, garnished with grated Parmesan cheese if desired.