Heat olive oil in a large pot over medium-high heat. Add diced onion and sauté until tender, about 3 minutes. Stir in sliced mushrooms and cook for another 3 minutes, then add minced garlic, Italian seasoning, salt, and black pepper. Cook for 30 seconds until fragrant.

Add cooked meatballs, chicken broth, and spaghetti sauce into the pot. Stir well and bring to a boil. If using frozen meatballs, allow extra time to fully heat.

Once boiling, gently stir in tortellini. Reduce heat to medium and cook according to package instructions, about 7 minutes, or until tortellini are al dente.

Taste and adjust seasoning. Ladle soup into bowls and garnish with chopped parsley or basil and grated Parmesan cheese. Serve hot.