In a large pot or Dutch oven, heat olive oil over medium-high heat. Add the onion and cook until it's starting to soften, about 2-3 minutes. Add the garlic and cook, stirring constantly, for 1 more minute. Stir in the diced tomatoes, tomato sauce, Italian seasoning, beef broth, and meatballs. Bring to a boil; reduce the heat to low and simmer (uncovered) for 20 minutes.
Add the tortellini and cook just until tender, about 5-7 more minutes. Stir in Parmesan cheese. Taste and season with salt and pepper, if necessary.
Serve soup immediately. Ladle into bowls and garnish with additional Parmesan cheese and fresh herbs, if desired.
