In a pot, heat the olive oil over medium heat. Saute the onion and garlic; until softened and slightly brown, about 6 minutes. Pour in the broth and bring to a simmer.
Add in the meatballs and simmer for 10 minutes. Then add in the spinach.
Slowly pour in the egg while whisking the soup in a circular motion to prevent the egg from clumping up. Allow to simmer for another 2 to 3 minutes.
Remove from heat, Stir in the parmesan cheese. Taste and adjust the seasoning with salt if needed.