In a bowl, combine turkey, garlic, egg, breadcrumbs, parmesan, salt and 1/4 tsp pepper. Using 1 tablespoon for each, roll mixture into meatballs.
In a large pot, heat oil over medium heat. Cook onion, stirring occasionally, until softened, 3-4 minutes. Add broth and tomatoes with juice; bring to a simmer. Add meatballs; cook, without stirring, until meatballs float to the surface, about 5 minutes.
Add as much spinach to the pot as will fit. Cook, gradually adding remaining spinach, until wilted and meatballs are cooked thru, about 5 minutes more. Thin soup with water or more broth if desired; season with salt and pepper. Serve soup sprinkled with more Parmesan.